This change exposes a new type, Match, which represents a matched route.
When present in the Goji environment (bound to the key MatchKey),
routing will be skipped and the bound Match will be dispatched to
instead.

In addition, the Goji router has been exposed as a middleware using the
Match mechanism above. This allows middleware inserted after the router
access to the Match object and any bound URLParams.

Fixes #76. See also #32.