Signals: before_action & after_action

It would be a nice thing to have signals, or something close to signals.
If you look at Flask, for example, there exist a `before_request` function which is executed before each request
http://flask.pocoo.org/docs/api/#flask.Flask.before_request
Here you can look at a simple suggestion of calling `before_action` (if it's define), then the actual `action`, and after that `after_action`.