Add visual regression testing

[jamesplease]

Pieces to put together:

1. Jest
2. jest-image-snapshot
3. Puppeteer
4. Storybook with the UI hidden

I may be underestimating the challenge, but I think that this should be straightforward. The hardest problem I can think of is the npm script that:

• starts the storybook
• once it is running, runs the tests
• shuts the storybook down when the tests are complete
• fails when the test fails, succeeds when the test succeeds