Support .yml extension in addition to .yaml

[EvanLovely]

It's great that data files can take the form of pattern.yaml, but many people use it as pattern.yml – even Pattern Lab's own config/config.yml! This can be a subtle difference and could easily add some confusion for users.

[dmolsen]

Easy enough to fix. References in pattern rules should be json||yaml||yml instead of json||yaml. See PatternInfoRule.php as an example. I'll get to this late next week unless you get to this first. Finishing up #14 is my priority at the moment.