imp(Usage Strings): improves the default usage string when only a single positional arg is present

Instead of blindly printing `[ARGS]` when only a single positional arg is present, it will now
print `[NAME]` (or `[NAME]...` for multiple values allowed)

Closes #518

src/app/parser.rs

@@ -1514,7 +1514,17 @@ impl<'a, 'b> Parser<'a, 'b>
             }
             if self.has_positionals() &&
                self.positionals.values().any(|a| !a.settings.is_set(ArgSettings::Required)) {
-                usage.push_str(" [ARGS]");
+                if self.positionals.len() == 1  {
+                    let p = self.positionals.values().next().expect(INTERNAL_ERROR_MSG);
+                    if !self.groups.values().any(|g| g.args.iter().any(|a| a == &p.name)) {
+                        usage.push_str(&*format!(" [{}]{}", p.name_no_brackets(),
+                        p.multiple_str()));
+                    } else {
+                        usage.push_str(" [ARGS]");
+                    }
+                } else {
+                    usage.push_str(" [ARGS]");
+                }
             }
 
 

src/args/arg_builder/positional.rs

@@ -1,6 +1,7 @@
 use std::fmt::{Display, Formatter, Result};
 use std::result::Result as StdResult;
 use std::rc::Rc;
+use std::borrow::Cow;
 
 use vec_map::VecMap;
 
@@ -108,6 +109,25 @@ impl<'n, 'e> PosBuilder<'n, 'e> {
         }
         pb
     }
+
+    pub fn multiple_str(&self) ->  &str {
+        if self.settings.is_set(ArgSettings::Multiple) && self.val_names.is_none() {
+            "..."
+        } else {
+            ""
+        }
+    }
+
+    pub fn name_no_brackets(&self) -> Cow<str> {
+        if let Some(ref names) = self.val_names {
+            Cow::Owned(names.values()
+                 .map(|n| format!("<{}>", n))
+                 .collect::<Vec<_>>()
+                 .join(" "))
+        } else {
+            Cow::Borrowed(self.name)
+        }
+    }
 }
 
 impl<'n, 'e> Display for PosBuilder<'n, 'e> {