Allow to restrict authorization at the plugin level.

[rnaud]

I am adding lita as a bot to our whole org but I want to be able to restrict access to some critical plugins (github or CodeDeploy integrations) to the engineers.

Currently I believe I need to fork these gems to restrict the routes to the right Lita group. Or is there a way to do this at a global level that I missed.

If this does not exist I'll take a jab at a PR for this.

[rnaud]

Hmmm, turns out route class methods are being called before the config for the handler is initialized, making it pretty hard currently to extend this. Ideas welcome 👍

[rnaud]

Took a pass at it but not very happy with it: #209