A stackable dynamic tree based Rack router.

Rack::Mount supports Rack's +X-Cascade+ convention to continue trying
routes if the response returns +pass+.  This allows multiple routes to
be nested or stacked on top of each other.  Since the application
endpoint can trigger the router to continue matching, middleware can
be used to add arbitrary conditions to any route.  This allows you to
route based on other request attributes, session information, or even
data dynamically pulled from a database.