Add auto-correction to RedundantSelf cop
Refactor add_offence and auto-correct logic

* introduce `convention` and `warning` wrappers around add_offence
* add node to the offence info
* call `autocorrect_action` from `add_offence` instead of explicitly
Fix a typo
Add cop MultilineBlockChain.
Refactor use of highlights method in specs.

Move to Cop#highlights defined in spec_helper.rb.