WIP: Introduce Rubocop to ensure code style 

References discussion in #2499 and includes code from #2500

In short this aims to correct inconsistencies in Jekyll's current code style and provide a means of checking code style is maintained in future contributions. Jekyll will be following the [GitHub Ruby Styleguide](https://github.com/styleguide/ruby).

- [ ] TODO: this todo list :sunglasses:
We'll go with what you have! I'll delete my other comments. Thanks!

(I looked through the source code, and any variable which starts with a `_` will not be flagged for a WARN about it being unused.)