Add an exit handler

Closes #243 

Adds the `exitHandler` service as well as the `exit` directive.

If a an exitHandler property is present in a conf file, the directive will be active and allow exiting to a specified place. The property is defined just like our resources.

The Perseids conf files already implement this - check them for more info on the syntax.

Note that the directive so far has no watchers at all - this means that a route change to a different conf file might lead to unexpected behavior when no page reload is performed. This can only happen though when users change the url to different conf files in their browser manually - probably nothing that should bother us too much for now.