Style: add rubocop

[DirtyF]

👋 This aims at relying on Rubocop to fix style issues

• [x] basic configuration for Rubocop
• [x] run safe auto-correct to fix offenses
• [x] run auto-gen-config to list remaining offenses

[Floppy]

Thanks! By coincidence I'm just making a few changes and will roll a release shortly, so I'll get this lot merged in.