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!
Additional tests for cookbook generator

Also added a strip call to the keys and values, as `generator_context. key` is not a legal identifier (due to the space).
Add documentation example for --generator-arg
Merge pull request #217 from martinb3/generator_arg

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