feat(HELP): Add a Templated Help system.

The strategy is to copy the template from the the reader to wrapped stream
until a tag is found. Depending on its value, the appropriate content is copied
to the wrapped stream.
The copy from template is then resumed, repeating this sequence until reading
the complete template.

Tags arg given inside curly brackets:
Valid tags are:
    * `{bin}`         - Binary name.
    * `{version}`     - Version number.
    * `{author}`      - Author information.
    * `{usage}`       - Automatically generated or given usage string.
    * `{all-args}`    - Help for all arguments (options, flags, positionals arguments,
                        and subcommands) including titles.
    * `{unified}`     - Unified help for options and flags.
    * `{flags}`       - Help for flags.
    * `{options}`     - Help for options.
    * `{positionals}` - Help for positionals arguments.
    * `{subcommands}` - Help for subcommands.
    * `{after-help}`  - Help for flags.

benches/04_new_help.rs

@@ -341,3 +341,27 @@ fn example10_new(b: &mut Bencher) {
     let app = example10();
     b.iter(|| build_new_help(&app));
 }
+
+#[bench]
+fn example4_template(b: &mut Bencher) {
+/*
+MyApp 1.0
+Kevin K. <[email protected]>
+Parses an input file to do awesome things
+
+USAGE:
+    test [FLAGS] <input>
+
+FLAGS:
+    -c, --config     sets the config file to use
+    -d, --debug      turn on debugging information
+    -h, --help       Prints help information
+    -V, --version    Prints version information
+
+ARGS:
+    <input>    the input file to use
+*/
+
+    let app = example4().template("{bin} {version}\n{author}\n{about}\n\nUSAGE:\n    {usage}\n\nFLAGS:\n{flags}\n\nARGS:\n{args}\n");
+    b.iter(|| build_new_help(&app));
+}