Feature Request: Auto-detect policy document fragment format

[vito-laurenza-zocdoc]

As a user, I should be able to write my IAM policy statements in either JSON or YAML and the tool should automatically handle either format.

Ideally this would be a distinct feature from the --format flag introduced in #13 which IMO should be refactored to influence the CloudFormation output format only.

[apmclean]

I imagine we could put a try: together that tried to pass the output from the jinja processing first to JSON, then to YAML, only failing if both reject. The --format flag could influence the output formatting only if the auto-detection worked as expected. Will have a look when I've got some cycles.