Add optional Guard development workflow

[dafyddcrosby]

I've found test-driven Cop development lends itself well to rapid feedback, and while I've hacked together nasty scripts up to now, a Guardfile that's set up for Cookstyle development would be mighty slick.

Motivation

As a Cookstyle developer working on a git checkout
I want to have Guard watch my changes
So that I can write high-quality Cops faster

Specification

Gemfile: optional gem group to install guard and guard-rake gems

Guardfile: watch for changes in /lib, kick off rspec (and maybe rubocop) runs as needed

Downstream Impact

Gem install is optional, so would not be an addition to chef-workstation. The guard gem is already packaged with chef-workstation, but guard-rake isn't, so that would have to be added to the omnibus/hab build stuff, and I don't want to create more work for @tpowell-progress 😄