Allow swagger files that use YAML extensions.

[taylor-sutton]

For example, at Clever make use of YAML merge keys and the ability to do YAML anchors and aliases out of order. This lets us keep our specs orderly with endpoints at the top and definitions at the bottom while still having endpoints refer to definitions in the logical way.

Not sure if this really belongs in wag or if we should just handle it client-side as we currently do. But maybe?