limit handling in url() method

[bglusman]

The stylus url method's base64 encoding option takes a limit parameter which defaults to 30kb/30000, but the behavior for files exceeding this limit is undefined; The current behavior is to pass larger files through as local file ref's, but the generated css file may not be served from the same location it's generated from (as in our case), making those references invalid. I'm considering developing a stylus plugin to change this behavior, but it might be nice/simpler to simply have an option or flag for erroring loudly or something when files above this limit are referenced, so they can be manually uploaded and handled as absolute URL's instead of relative.