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.
Also, it appears `yaml.safe_load`, doesn't support any kind of "strict" mode (`safe_load` is as strict as it gets). It will still parse some simple Python objects (dicts, lists, etc.):

From docs:

"Note that the ability to construct an arbitrary Python object may be dangerous if you receive a YAML document from an untrusted source such as the Internet. The function yaml.safe_load limits this ability to simple Python objects like integers or lists."