PUP-1661 Skeleton face implementation

So, as part of a larger feature of being able to define skeletons from external sources (https://tickets.puppetlabs.com/browse/PUP-1661) I've implemented a new face for the module command.

Right now, all the face does is list your current skeletons, but in the future it'll be able to take remote or local paths to add to your skeleton collection for module generation. It was all @apenney's idea :smile: 

This means less churn in adding in new files for the module template all the time.

So I've opened this PR to get some general advice on implementing this new face for the Module tool. Any help appreciated :+1: