Add `--json-attribs` option to generator base

Closes https://github.com/opscode/chef-dk/issues/210, adds an option to the base.rb generator command so that it accepts an arbitrary json file containing additional attributes that can be used to alter the behavior of the generator cookbook.

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!