Allow registration of custom global arguments #5649

On WordPress.com, we'd like to add --blog_id=<id> as a global argument. WordPress.com has all sorts domain transformation logic in a long sunrise.php so, in certain cases, it would be more reliable to pass a specific --blog_id=<id> argument.

The idea was originally discussed in #854 (comment)

Because --blog_id=<id> still requires some blog id -> URL lookup, I'm still not convinced it's something WP-CLI should be responsible for. However, third-party code (via sunrise.php or similar) could easily perform this logic, so it would be nice if it was possible to register --blog_id=<id> as a global argument.

A couple of ideas:

1. Have some form of filtering mechanism for the return values of WP_CLI::get_configurator():

   wp-cli/php/class-wp-cli.php

   Lines 61 to 69 in 0c427c5

   	public static function get_configurator() {
   	static $configurator;
   	if ( ! $configurator ) {
   	$configurator = new Configurator( WP_CLI_ROOT . '/php/config-spec.php' );
   	}
   	return $configurator;
   	}

2. Allow a custom path to be defined for config-spec.php:

   wp-cli/php/class-wp-cli.php

   Line 65 in 0c427c5

   $configurator = new Configurator( WP_CLI_ROOT . '/php/config-spec.php' );

I kind of like the second option, which would give the environment control over which global arguments were available. I'm not sure what breaks when you remove one, though.