Router "middleware"

[zenazn]

I plan on exposing the Goji router as a "middleware" layer of a sort: when this special middleware layer is called, it performs routing logic and places the resulting handler (and likely some other associated data about e.g., the handler that was chosen) into the Goji context. Finally, at the end of the middleware stack, if a route has already been chosen, it will be dispatched to (otherwise, Goji will behave exactly as it currently does).

The motivating use case here is to be able to write middleware that have access to bound URL parameters, or to allow a request logger to print information about which route was chosen for any given request.

Based on a discussion with @bradrydzewski in #32.

[zenazn]

Also #70

[pkieltyka]

+1