Use Rubocop

The initial rubocop config checks were pulled from rspec/rspec-expectations#544. I've added an exclude section with comments for which checks are being disabled for specific files and why.

Additionally, I'm only checking the `lib/` directory right now. It may be good to check a few other directories in the future (i.e. `features/`, `templates/`), but I'm not sure just yet.

Resolve #1072