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've been considering this function name. If you check out the other bits of hubot's API, they are like `respond` and `hear`, not `addRespond` and `addHear`, so it doesn't seem consistent with those.

I'm not sure it can be helped though, because if it was `listenerMiddleware`, it'd presumably clash with the rename I proposed for `middleware` -> `listenerMiddleware`. Annoying pluralization of middleware :sweat: