Add Rubocop to the project

Those of us that were at the sprint in May will remember that Hound CI was brought up because we wanted to try and automate the use of Rubocop. However, we completely forgot to add Rubocop to the project.

Hound does style checking in the PR, but that can be incredibly noisy ~~sometimes~~ a lot of the time. Though we cannot force someone to use Rubocop before opening a PR, having Rubocop in the default project configuration might act as a reminder.

I've linked the Hound and Rubocop configuration files, so you _should_ be able to just run `rubocop` on a file or directory and get the same results that Hound would give you. In practice, the results might be different due to the Gemfile locking onto an older version of Rubocop than what Hound is using.
Is this the syntax for including another YAML file? Where can I find the documentation about this?

I've seen Rubocop using the `inherit_from` key for inheritance, but, quoting from Hound's website, 
> [...] most of the RuboCop configuration, except inheritance, is supported in Hound as well.
So I was under the impression that we need to put everything in `.hound.yml`, and use `inherit_from` in `.rubocop.yml`.