Easier instrumentation via env['sinatra.route']

Adds a `sinatra.route` entry to `request.env` with the route that was matched for the current request.

This allows an after filter or rack middleware to gather per-route statistics about incoming requests.

For example, newrelic_rpm could use this feature instead of [scanning all the routes a second time](https://github.com/newrelic/rpm/blob/master/lib/new_relic/agent/instrumentation/sinatra.rb#L44-L53).