Use correct file extension for yaml

[adius]

The official file extension is .yaml: http://www.yaml.org/faq.html

It should be the default (but of course you should support .yml as well)

[doowb]

A PR to add that is welcome to add the mapping.

When the refactor of this lib is done, I'll keep that in mind, but it'll probably be easier to configure instead of hardcoding it.