Introduce a 'link_request' directive.

This turned out to be quite easy :) - I hope I didn't overlook anything.

Also I couldn't really explain to you what `LinkPrefixDirective` actually does. I picked this from looking at the other directives, but I didn't spend too much time on it, so maybe I did something horribly wrong there.

This PR is related to #185.
I can't get this to work (or I am getting something wrong). This always returns `http://mounted/`, when it should return `http://app/`, right?