- set `__mountpath` in `.use()`
- override `process_params` to add `req.matchedRoutes`

`req.matchedRoutes` is a proposed addition to express v5:
pillarjs/router#34

Doing this because in goodeggs-stats, we need to know what route we are
currently on when we are publishing metrics to librato. In express v4
there is no mechanism to do this, if the routes have parameters in them
(like :id, etc) or if they have nesting (given /foo/bar, the middleware
for /foo has no knowledge of /bar and vice-versa).