Add a new cop

Use a rake task to generate a cop template.

$ bundle exec rake new_cop[Category/Name]
- lib/rubocop/cop/category/name.rb
- spec/rubocop/cop/category/name_spec.rb
- `require 'lib/rubocop/cop/category/name'` added into lib/rubocop.rb

Do 3 steps
- Add an entry to `New feature` section in
  - e.g. Add new `Category/Name` cop. ([@your_id][])
- Add an entry into config/enabled.yml or config/disabled.yml
- Implement a new cop to the generated file!