Add per-room permissions for plugins

Blacklist or whitelist specific plugins by classname for specific rooms with a new "permissions" config option. By default the permissions model is permissive; if the "permissions" config attribute is missing or a room does not have permissions configured, all plugins will be active... which means this can be merged and current installations will be unaffected. 

We should really add testing to this project. I feel dirty making a pull request without tests!