Make --http=<domain> a global argument

[danielbachhuber]

Currently, we have to register --http=<domain> as an assoc argument so WP-CLI's synopsis parser doesn't error on it:

$synopsis[] = array(
    'name'        => 'http',
    'type'        => 'assoc',
    'optional'    => true,
);

It would be better to be able to register --http=<domain> as a global argument, in such a way it doesn't get treated as a command argument (both with the synopsis parser, and passed through as $assoc_args)

From #8