Add skeleton option

Not passing yet because Mixlib::CLI is confusing me with execution
context and wrapping parse options.
Move skeleton option into module
Cleanup and rename to generator_cookbook

This is per discussion in the Pull Request as skeleton may not be the
best name for this feature. Also did related cleanup around the naming
to keep things more consistent.
Add missing merge options from previous commit
Rename DIR to PATH

This will be a bit more consistent with the existing code farther down
the module.
Update CHANGELOG for generator-cookbook
Merge pull request #40 from martinisoft/add_skeleton_option

Add skeleton option for chef generate