[Mono.Options] Add option header support.

[Mono.Options] Add option header support.

Option headers can be provided via OptionSet.Add(string), and allows
forgoing the use of a separate mechanism to provide contextual output.

For example, one used to do this:

	bool show_help = false;
	new OptionSet  {
		// ...
		{ "help|h|?", v => show_help = v != null },
	}.Parse (args);

	if (show_help) {
		Console.WriteLine ("usage: sample-app ARGS");
		p.WriteOptionDescriptions (Console.Out);
		// ...
	}

Using the new header support, the "contextual" output can be merged
with the option specification:

	bool show_help = false;
	new OptionSet {
		"usage: sample-app ARGS",
		{ "help|h|?", v => show_help = v != null },
	}.Parse (args);

	if (show_help)
		p.WriteOptionDescriptions (Console.Out);

Furthermore, headers can be provided "inline", thus serving as a
mechanism to categorize options:

	// mcs options
	var p = new OptionSet {
		"Mono C# compiler, Copyright 2001 - 2011 Novell, Inc.",
		"mcs [options] source-files",
		// ...
		"Resources:",
		{ "linkresource|linkres:", v => /* ... */ },
		// ...
		"",
		"Options can be of the form -option or /option",
	};