Add `--generator-arg` option to generator base

Closes #210, adds an option to the base.rb generator command so that it
accepts an arbitrary key=value pairs that can be used to alter the behavior of
the generator cookbook. Spoke with @danielsdeleo about this solution being an
improvement over https://github.com/opscode/chef-dk/pull/211.

I didn't add any additional unit tests, as it doesn't appear the current set
of commandline arguments were being tested systematically. If you all approve
of this change, I'd be glad to add additional testing. Thank you!