Don't run validations for nothing

Since some projects have hundreds of files to validates, it is becoming a real waste of time to wait after Rubocop to check every files. Even those who haven't been touched. This is the raw prototype, to present you the basic idea. If that's something we want, I'll apply it to the other validators.

By default, it will behave as usual. To use this feature, add the `--diff` options.

:clock1: :moneybag: