feat(Arg): adds new setting Arg::require_delimiter which requires val delimiter to parse multiple values

Using this setting requires a value delimiter be present in order to parse multiple values.
Otherwise it is assumed no values follow, and moves on to the next arg with a clean slate.

These examples demonstrate what happens when `require_delimiter(true)` is used. Notice
everything works in this first example, as we use a delimiter, as expected.

```rust
let delims = App::new("reqdelims")
    .arg(Arg::with_name("opt")
        .short("o")
        .takes_value(true)
        .multiple(true)
        .require_delimiter(true))
    // Simulate "$ reqdelims -o val1,val2,val3"
    .get_matches_from(vec![
        "reqdelims", "-o", "val1,val2,val3",
    ]);

assert!(delims.is_present("opt"));
assert_eq!(delims.values_of("opt").unwrap().collect::<Vec<_>>(), ["val1", "val2", "val3"]);
```
In this next example, we will *not* use a delimiter. Notice it's now an error.

```rust
let res = App::new("reqdelims")
    .arg(Arg::with_name("opt")
        .short("o")
        .takes_value(true)
        .multiple(true)
        .require_delimiter(true))
    // Simulate "$ reqdelims -o val1 val2 val3"
    .get_matches_from_safe(vec![
        "reqdelims", "-o", "val1", "val2", "val3",
    ]);

assert!(res.is_err());
let err = res.unwrap_err();
assert_eq!(err.kind, ErrorKind::UnknownArgument);
```
What's happening is `-o` is getting `val1`, and because delimiters are required yet none
were present, it stops parsing `-o`. At this point it reaches `val2` and because no
positional arguments have been defined, it's an error of an unexpected argument.

In this final example, we contrast the above with `clap`'s default behavior where the above
is *not* an error.

```rust
let delims = App::new("reqdelims")
    .arg(Arg::with_name("opt")
        .short("o")
        .takes_value(true)
        .multiple(true))
    // Simulate "$ reqdelims -o val1 val2 val3"
    .get_matches_from(vec![
        "reqdelims", "-o", "val1", "val2", "val3",
    ]);

assert!(delims.is_present("opt"));
assert_eq!(delims.values_of("opt").unwrap().collect::<Vec<_>>(), ["val1", "val2", "val3"]);
```

src/app/parser.rs

@@ -1256,7 +1256,7 @@ impl<'a, 'b> Parser<'a, 'b>
                         ret = try!(self.add_single_val_to_arg(arg, v, matcher));
                     }
                     // If there was a delimiter used, we're not looking for more values
-                    if val.contains_byte(delim as u32 as u8) {
+                    if val.contains_byte(delim as u32 as u8) || arg.is_set(ArgSettings::RequireDelimiter) {
                         ret = None;
                     } 
                 }

src/args/arg.rs

@@ -1919,6 +1919,62 @@ impl<'a, 'b> Arg<'a, 'b> {
         }
     }
 
+    /// Specifies whether or not an argument should allow grouping of multiple values via a
+    /// delimiter. I.e. should `--option=val1,val2,val3` be parsed as three values (`val1`, `val2`,
+    /// and `val3`) or as a single value (`val1,val2,val3`). Defaults to using `,` (comma) as the
+    /// value delimiter for all arguments that accept values (options and positional arguments)
+    ///
+    /// **NOTE:** The default is `true`. Setting the value to `true` will reset any previous use of
+    /// [`Arg::value_delimiter`] back to the default of `,` (comma).
+    ///
+    /// # Examples
+    ///
+    /// The following example shows the default behavior.
+    ///
+    /// ```rust
+    /// # use clap::{App, Arg};
+    /// let delims = App::new("delims")
+    ///     .arg(Arg::with_name("option")
+    ///         .long("option")
+    ///         .takes_value(true))
+    ///     .get_matches_from(vec![
+    ///         "delims",
+    ///         "--option=val1,val2,val3",
+    ///     ]);
+    ///
+    /// assert!(delims.is_present("option"));
+    /// assert_eq!(delims.occurrences_of("option"), 1);
+    /// assert_eq!(delims.values_of("option").unwrap().collect::<Vec<_>>(), ["val1", "val2", "val3"]);
+    /// ```
+    /// The next example shows the difference when turning delimiters off.
+    ///
+    /// ```rust
+    /// # use clap::{App, Arg};
+    /// let nodelims = App::new("nodelims")
+    ///     .arg(Arg::with_name("option")
+    ///         .long("option")
+    ///         .use_delimiter(false)
+    ///         .takes_value(true))
+    ///     .get_matches_from(vec![
+    ///         "nodelims",
+    ///         "--option=val1,val2,val3",
+    ///     ]);
+    ///
+    /// assert!(nodelims.is_present("option"));
+    /// assert_eq!(nodelims.occurrences_of("option"), 1);
+    /// assert_eq!(nodelims.value_of("option").unwrap(), "val1,val2,val3");
+    /// ```
+    /// [`Arg::value_delimiter`]: ./struct.Arg.html#method.value_delimiter
+    pub fn require_delimiter(mut self, d: bool) -> Self {
+        if d {
+            self.setb(ArgSettings::UseValueDelimiter);
+            self.set(ArgSettings::RequireDelimiter)
+        } else {
+            self.unsetb(ArgSettings::UseValueDelimiter);
+            self.unset(ArgSettings::RequireDelimiter)
+        }
+    }
+
     /// Specifies the separator to use when values are clumped together, defaults to `,` (comma).
     ///
     /// **NOTE:** implicitly sets [`Arg::use_delimiter(true)`]

src/args/settings.rs

@@ -3,15 +3,16 @@ use std::ascii::AsciiExt;
 
 bitflags! {
     flags Flags: u16 {
-        const REQUIRED       = 0b000000001,
-        const MULTIPLE       = 0b000000010,
-        const EMPTY_VALS     = 0b000000100,
-        const GLOBAL         = 0b000001000,
-        const HIDDEN         = 0b000010000,
-        const TAKES_VAL      = 0b000100000,
-        const USE_DELIM      = 0b001000000,
-        const NEXT_LINE_HELP = 0b010000000,
-        const R_UNLESS_ALL   = 0b100000000,
+        const REQUIRED       = 0b0000000001,
+        const MULTIPLE       = 0b0000000010,
+        const EMPTY_VALS     = 0b0000000100,
+        const GLOBAL         = 0b0000001000,
+        const HIDDEN         = 0b0000010000,
+        const TAKES_VAL      = 0b0000100000,
+        const USE_DELIM      = 0b0001000000,
+        const NEXT_LINE_HELP = 0b0010000000,
+        const R_UNLESS_ALL   = 0b0100000000,
+        const REQ_DELIM      = 0b1000000000,
     }
 }
 
@@ -33,7 +34,8 @@ impl ArgFlags {
         TakesValue => TAKES_VAL,
         UseValueDelimiter => USE_DELIM,
         NextLineHelp => NEXT_LINE_HELP,
-        RequiredUnlessAll => R_UNLESS_ALL
+        RequiredUnlessAll => R_UNLESS_ALL,
+        RequireDelimiter => REQ_DELIM
     }
 }
 
@@ -67,6 +69,8 @@ pub enum ArgSettings {
     UseValueDelimiter,
     /// Prints the help text on the line after the argument
     NextLineHelp,
+    /// Requires the use of a value delimiter for all multiple values
+    RequireDelimiter,
     #[doc(hidden)]
     RequiredUnlessAll,
 }
@@ -84,6 +88,7 @@ impl FromStr for ArgSettings {
             "usevaluedelimiter" => Ok(ArgSettings::UseValueDelimiter),
             "nextlinehelp" => Ok(ArgSettings::NextLineHelp),
             "requiredunlessall" => Ok(ArgSettings::RequiredUnlessAll),
+            "requiredelimiter" => Ok(ArgSettings::RequireDelimiter),
             _ => Err("unknown ArgSetting, cannot convert from str".to_owned()),
         }
     }