Add skeleton option for chef generate

This is a Pull Request to add in a `--skel` option as it's documented in the methods for the generators. I'd like to have this to use custom skeleton cookbooks for generating different types of cookbooks. Eventually I'd like to re-factor this a bit so it can simply command the skeleton cookbook to run a given recipe argument giving way to fully custom generators based upon a skeleton cookbook.

This is not passing yet because Mixlib::CLI is confusing me with execution context and wrapping parse options. Can anyone provide some guidance on this for me?