Add skeleton option for chef generate

I'd like to add an option to the `chef generate` command to allow customization of the 'skeleton' cookbook path. Currently it is hard-wired in to the gem and it seems the option is documented in the methods for the generators [here](https://github.com/opscode/chef-dk/blob/b6de1a231234b54f604107e8e6fcb680458b6946/lib/chef-dk/command/generator_commands.rb#L91) and [here](https://github.com/opscode/chef-dk/blob/b6de1a231234b54f604107e8e6fcb680458b6946/lib/chef-dk/command/generator_commands.rb#L154). 

Eventually I'd like to re-factor this a bit so it can simply command the specified generator cookbook in a path to run a given recipe argument giving way to fully custom generators based upon a cookbook. Also, I think it's really cool that this is using a cookbook to build cookbooks :turtle:.