Add support for yaml

[lucaslago]

It would be nice if we added support to yaml defined swagger spec

[earldouglas]

Great idea!

[sunapi386]

Should be straight forward, open as yaml and dump to json, still the same pipeline. Although doing it right means supporting split yaml files (and json files), as such https://azimi.me/2015/07/16/split-swagger-into-smaller-files.html

[radeklat]

For command line use, it would be good to add to the README a simple workaround: python -c 'import sys, yaml, json; json.dump(yaml.load(sys.stdin), sys.stdout)' < swagger.yaml | swagger-test