Support CloudFormation snippets as input?

[flosell]

This was raised in #48 where the @joemilacek tried to input a snippet of CloudFormation JSON which is currently not supported (we only support plain IAM policy documents).
However, the use-case doesn't seem too far-fetched. It's likely a significant piece of work though so I probably won't work on it unless there's significant need for this feature.

As a start, we could introduce a specific error message that allows us to put analytics on how often this use-case happens.

If you have additional comments on the need for this feature, feel free to comment below.