Add Hound (style checker)

[benjaminoakes]

To have a consistent style, we should add Hound which checks style on a pull request.

Acceptance Criteria

• [x] Link Hound with this repo (Done! See status checks on #20 for an example.)
• [x] Add any necessary configuration. See their docs for more details.
  • Stick to default styles as much as possible (whatever Hound is configured to do by default) (Turns out we don't need a config file.)
• [x] Make a pull request with style violations to confirm that everything works. (Done! See #25.)

[benjaminoakes]

I took care of this so that new PRs can be checked automatically.