add _matchedRoute to context

[jergason]

Expose the route that matched the current request on context. This is useful for monitoring and analytics, and enables you to do something like the following:

function* routeMetrics(next) {
  var start = Date.now()
  yield next
  var duration = Date.now() - start
  // send a metric
  statsd.timing(this._matchedRoute, duration)
  // this.matchedRoute would be the string of the matched route, so '/foo/:id/:type/' in this case
}

This will send a count of the number of times a specific route is hit, even if the route has dynamic parts.

I'm happy to tweak this, change the name, or add more tests if you are ok with this basic idea.

I believe this fixes #223.

[kunalgolani]

if you add another similar but more generic route here, the test will fail for the above route

router.get('/:username/:id', function *() {
    // this._matchedRoute for '/users/:id' will now instead be '/:username/:id'
})