[Mono.Options] Add Mono.Options.Command, .CommandSet

[Mono.Options] Add Mono.Options.Command, .CommandSet

Mono.Options.CommandSet allows easily having separate commands and
associated command options, allowing creation of a *suite* along the
lines of **git**(1), **svn**(1), etc.

CommandSet allows intermixing plain text strings for `--help` output,
Option values -- as supported by OptionSet -- and Command instances,
which have a name, optional help text, and an optional OptionSet.

	var suite = new CommandSet ("suite-name") {
		// Use strings and option values, as with OptionSet
		"usage: suite-name COMMAND [OPTIONS]+",
		{ "v:", "verbosity", (int? v) => Verbosity = v.HasValue ? v.Value : Verbosity+1 },
		// Commands may also be specified
		new Command ("command-name", "command help") {
			Options = new OptionSet {/*...*/},
			Run     = args => { /*...*/},
		},
		new MyCommandSubclass (),
	};
	suite.Run (new string[]{...});

CommandSet provides a `help` command, and forwards `help COMMAND`
to the registered Command instance by invoking Command.Invoke()
with `--help` as an option.