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.
You need to modify view() to add the right code:

```     
         old_app = self.app
+       old_lookup = self.lookup
         app.set_implicit()
         self.app = app
+       self.lookup = app.lookup
         result = view(self, obj)
         old_app.set_implicit()
         self.app = old_app
+        self.lookup = old_lookup
         return result
```

Apply this to your patch. Then we you can move on to implement the caching strategy I sketched out before (as I do think we need to cache the prefix instead of recalculating it each time), and merge it.