Add an optional delegate to URLRouter.match

This allow one to fine tune the way request are handled (e.g., based on ContentType, Accept, If-*...).

This is already possible, but has to be done inside the called handler.
The called handler shouldn't have to check if it is treating the correct information, that's the router job.
Also, this might is needed by generators (such as vibe.web.rest), to implement attributes such as `@Consumes` and `@Produces`, as found in Java Jersey.