Introduce RuboCop to ensure code style

[RuboCop](https://github.com/bbatsov/rubocop) is a Ruby static code analyzer for consistency of code style. Inch will be following [Ruby Style Guide](https://github.com/bbatsov/ruby-style-guide).

I generated [.rubocop_todo.yml](https://github.com/yous/inch/blob/patch-rubocop/.rubocop_todo.yml) by running `rubocop --auto-gen-config`, it disables specific cops that don't pass in the Inch code. The goal is empty this file.