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.
Btw, we should reverse the order of operations - try JSON first and YAML second.

The reason for that is that `yaml.safe_load` successfully parses almost any kind of string, even things which looks like JSON (e.g., try `yaml.safe_load('{"a": "d"}')`, `yaml.safe_load('a')`, etc.)