RFR: Support yaml metadata for actions and rules

* Add: Yaml support for actions and rules meta. Both yaml and yml extensions
are supported.
* Add: yaml, yml and json meta for sample action in fixtures for testing.
Looking at the code a couple of lines bellow (`rules = self._get_rules_from_pack(rules_dir)`), it looks like the argument is a directory, so calling the argument `pack` is slightly confusing and misleading.

Maybe we should call it `rules_directory` or something?