Add response file suport to Mono.Options.

Add response file suport to Mono.Options.

Response file support is an "opt-in" feature, performed by adding a
ResponseFileSource to the OptionSet initialization logic:

	var p = new OptionSet () {
		// normal options here...
		new ResponseFileSource (),
	};

Response files make use of the new "ArgumentSource" support, which
acts as a low-level argument pre-processor, allowing processing of
arguments _before_ Option processing is performed.