api: adds new App method calls for printing and writing the long version of help messages

One can now use `App::print_long_help` or `App::write_long_help`

Note that `App::write_long_help` is **NOT** affected by kbknapp/clap-rs#808 in the manner
that `App::write_help` help is and thus should be preferred if at all possible.

src/app/mod.rs

@@ -1095,7 +1095,11 @@ impl<'a, 'b> App<'a, 'b> {
         self
     }
 
-    /// Prints the full help message to [`io::stdout()`] using a [`BufWriter`]
+    /// Prints the full help message to [`io::stdout()`] using a [`BufWriter`] using the same
+    /// method as if someone ran `-h` to request the help message
+    ///
+    /// **NOTE:** clap has the ability to distinguish between "short" and "long" help messages
+    /// depending on if the user ran [`-h` (short)] or [`--help` (long)]
     ///
     /// # Examples
     ///
@@ -1106,6 +1110,8 @@ impl<'a, 'b> App<'a, 'b> {
     /// ```
     /// [`io::stdout()`]: https://doc.rust-lang.org/std/io/fn.stdout.html
     /// [`BufWriter`]: https://doc.rust-lang.org/std/io/struct.BufWriter.html
+    /// [`-h` (short)]: ./struct.Arg.html#method.help
+    /// [`--help` (long)]: ./struct.Arg.html#method.long_help
     pub fn print_help(&mut self) -> ClapResult<()> {
         // If there are global arguments, or settings we need to propgate them down to subcommands
         // before parsing incase we run into a subcommand
@@ -1119,7 +1125,45 @@ impl<'a, 'b> App<'a, 'b> {
         self.write_help(&mut buf_w)
     }
 
-    /// Writes the full help message to the user to a [`io::Write`] object
+    /// Prints the full help message to [`io::stdout()`] using a [`BufWriter`] using the same
+    /// method as if someone ran `-h` to request the help message
+    ///
+    /// **NOTE:** clap has the ability to distinguish between "short" and "long" help messages
+    /// depending on if the user ran [`-h` (short)] or [`--help` (long)]
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// # use clap::App;
+    /// let mut app = App::new("myprog");
+    /// app.print_long_help();
+    /// ```
+    /// [`io::stdout()`]: https://doc.rust-lang.org/std/io/fn.stdout.html
+    /// [`BufWriter`]: https://doc.rust-lang.org/std/io/struct.BufWriter.html
+    /// [`-h` (short)]: ./struct.Arg.html#method.help
+    /// [`--help` (long)]: ./struct.Arg.html#method.long_help
+    pub fn print_long_help(&mut self) -> ClapResult<()> {
+        // If there are global arguments, or settings we need to propgate them down to subcommands
+        // before parsing incase we run into a subcommand
+        self.p.propogate_globals();
+        self.p.propogate_settings();
+        self.p.derive_display_order();
+
+        self.p.create_help_and_version();
+        let out = io::stdout();
+        let mut buf_w = BufWriter::new(out.lock());
+        self.write_long_help(&mut buf_w)
+    }
+
+    /// Writes the full help message to the user to a [`io::Write`] object in the same method as if
+    /// the user ran `-h`
+    ///
+    /// **NOTE:** clap has the ability to distinguish between "short" and "long" help messages
+    /// depending on if the user ran [`-h` (short)] or [`--help` (long)]
+    ///
+    /// **NOTE:** There is a known bug where this method does not write propogated global arguments
+    /// or autogenerated arguments (i.e. the default help/version args). Prefer
+    /// [`App::write_long_help`] instead if possibe!
     ///
     /// # Examples
     ///
@@ -1131,6 +1175,8 @@ impl<'a, 'b> App<'a, 'b> {
     /// app.write_help(&mut out).expect("failed to write to stdout");
     /// ```
     /// [`io::Write`]: https://doc.rust-lang.org/std/io/trait.Write.html
+    /// [`-h` (short)]: ./struct.Arg.html#method.help
+    /// [`--help` (long)]: ./struct.Arg.html#method.long_help
     pub fn write_help<W: Write>(&self, w: &mut W) -> ClapResult<()> {
         // PENDING ISSUE: 808
         //      https://github.com/kbknapp/clap-rs/issues/808
@@ -1144,6 +1190,33 @@ impl<'a, 'b> App<'a, 'b> {
         Help::write_app_help(w, self, false)
     }
 
+    /// Writes the full help message to the user to a [`io::Write`] object in the same method as if
+    /// the user ran `--help`
+    ///
+    /// **NOTE:** clap has the ability to distinguish between "short" and "long" help messages
+    /// depending on if the user ran [`-h` (short)] or [`--help` (long)]
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// # use clap::App;
+    /// use std::io;
+    /// let mut app = App::new("myprog");
+    /// let mut out = io::stdout();
+    /// app.write_long_help(&mut out).expect("failed to write to stdout");
+    /// ```
+    /// [`io::Write`]: https://doc.rust-lang.org/std/io/trait.Write.html
+    /// [`-h` (short)]: ./struct.Arg.html#method.help
+    /// [`--help` (long)]: ./struct.Arg.html#method.long_help
+    pub fn write_long_help<W: Write>(&mut self, w: &mut W) -> ClapResult<()> {
+        self.p.propogate_globals();
+        self.p.propogate_settings();
+        self.p.derive_display_order();
+        self.p.create_help_and_version();
+
+        Help::write_app_help(w, self, true)
+    }
+
     /// Writes the version message to the user to a [`io::Write`] object
     ///
     /// # Examples
@@ -1216,7 +1289,7 @@ impl<'a, 'b> App<'a, 'b> {
     /// build = "build.rs"
     ///
     /// [build-dependencies]
-    /// clap = "2.9"
+    /// clap = "2.23"
     /// ```
     ///
     /// Next, we place a `build.rs` in our project root.