Resolve implied URL to an absolute URL

Just like an implied photo is resolved to an absolute URL an implied URL should be resolved to one, so I copied the solution from photos and applied it to the implied URL:s as well.

Also double checked at http://microformats.org/wiki/microformats2-parsing#parsing_for_implied_properties that this is how it should be parsed.

Also have to say: Thanks for a great Node.js library! Likely wouldn't have done at all as much IndieWeb stuff without it lowering the barrier.