Add a `exit` function for app exit. 

It would be the opposite of `init` and would allow for cleanup tasks after the application has generated a response. 

The idea is to provide a hook for logic that should happen after having generated a response, like maintenance http calls or cache cleanup stuff.
I'm starting to regret not making events more nodejs-llike (`.emit('foo', {bar: 'baz'})`) when moving to v2.