Add basic arg validation - #178

My first though was to have a static get_accepted_args() function on each class:

static function get_accepted_args() {
  return array( 'foo', 'bar' );
}

Since it's not per-method, you can't specify required args, or you can, but verbosely.

So, what I'm thinking of now is to have a @synopsis adnotation:

/**
 * Create a blog in a multisite install.
 *
 * @subcommand create
 * @synopsis --slug=<slug> --title=<Title> [--email=<email>] [--site_id=<site-id>] [--public]
 */
public function create( $args, $assoc_args ) {

The @subcommand tag would be optional.