New Cop generators

[dafyddcrosby]

After the Cookstyle hack fest, the amount of boilerplate necessary to create a new Cop became apparent, and this seems like a problem worth solving.

Motivation

As a new Cookstyle dev,
I want to be able to write a "hello world" Cop easily
So that I'm able to get started on basic tasks without needing to read RuboCop docs for N hours

As an experienced Cookstyle dev,
I want to avoid writing boilerplate
So that I can write new Cops quickly

Specification

While I think the case of writing private Cops is worth investigating, to start this should be a Rake task that writes out:

• a Cop class skeleton
• a spec file skeleton
• adds the Cop to the cookstyle.yml file
• a documentation stub