Introduces 'capsule_controller_action' action

Adds a default clause at the end of the capsule_controller() switch and calls
do_action( 'capsule_controller_action', $_POST['capsule_action'] ) to allow
third party plugins to extend the core functionality creating new actions.
Merge pull request #6 from MZAWeb/feature/capsule-controller-action

Introduces 'capsule_controller_action' action