Listener Middleware

Builds on #801 (57b722022807b9a5befaafcab2442ad3a03d455b) to allow injecting arbitrary code between the Listener match and execute steps. Like in Express, middleware can interrupt the response process, preventing the Listener callback from ever being executed. Middleware can perform operations both on the way towards the Listener callback (before callback execution) and on the way away from the Listener callback (after callback execution/middleware interrupt).

As a side effect, listeners are now executed asynchronously. Behavior around message.done should remain the same (process until message.done is true).

Example usage (imposes authorization policy on all scripts): https://github.com/michaelansel/hubot-rbac/blob/master/src/rbac.coffee
I almost did that, but I was concerned about putting in a not-so-great example: if you send a message every time an incoming message is rate limited, you end up responding to a flood of messages with a flood of replies. Solving this isn't too difficult, but would require an additional output rate limiter. Maybe I'm being too picky about my example code...