add filter for content_url and plugins_url

Because `content_url()` and `plugins_url()` are still based on some default constants (see link-template.php [L#2798](https://core.trac.wordpress.org/browser/tags/4.1/src/wp-includes/link-template.php#L2798) and [L#2830](https://core.trac.wordpress.org/browser/tags/4.1/src/wp-includes/link-template.php#L2830)), we have to add your `mangle_url` filtert here also.

I think [this](https://github.com/humanmade/Mercator/blob/master/mercator.php#L289) would be a nice place.