Add ability to filter controller actions in controller.

Quite often you have a bit of logic duplicated between various controller actions. It's not ideal to make a real lithium filter since then the logic itself would exist in another file, or the logic to call a new method would exist in another file.

It was possible before to do before filters generically by making a filter on `Dispatcher::_callable`, however an after filter was not possible. You could potentially play with the entire response object using `Dispatcher::_call` but you could not change things that were `$controller->set()` from earlier.