Support rubocop --require option

Support rubocop --require option (rubocop [source](https://github.com/bbatsov/rubocop/blob/master/lib/rubocop/options.rb#L77)) so that we can use custom cops (e.g., [rubocop-rspec](https://github.com/nevir/rubocop-rspec#command-line)) by rubocop-git!