Support loading in Figfiles with .yaml extension

In response to issue #51, I've added support for using the `.yaml` file extension when working with Fig.

I'm not at all precious about the content of any logging messages I've added here, and I'm pretty rusty when it comes to using Python testing frameworks, so I'll happily take any pointers on which kinds of assertions you'd prefer here in future tests.

Cheers, 

Chris