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.
Yeah. I'm trying to reason through why this happens even though you made it a property (the intermediate solution I suggested). The link prefix should be recalculated each time, and ``request.view`` does change request.lookup to the parent app context, so why isn't it working? I'm mystified as well. I'll do some debugging...