Added filter hook for modifying the PageLinkTo (PLT) URL

Added a filter hook within the meta_link processing, so URLs parsed by PLT can be modified by an external filter listener prior to making their way onto the page.

This was used on a project to make PLT handle URLs like "http://$token$/some/page/location" and reprocess $token$ into a desired URL, but it could be used for all sorts of handy things.