Add c-s-c show-versions subcommand

We should have a way to output the versions of components present on the system.

Documented in https://github.com/opscode/chef-docs/pull/413
This is a small command, so I don't want to hold it up with bike shedding, but why not just do this in Ruby?  Also, do we really want to glob all versions? That could be pretty verbose output and it means that the ordering of the output can change based on the underlying filesystem and what add-on's you have installed.