Add Jest tests and Prettier support (hound-search#343)

This commit adds support for Jest and writes a few simple tests as
proof. Adding a test is as easy as adding a file that ends in
`.test.js` next to the file you want to test. Any file that matches that
pattern should have access to Jest functions. The whole test suite can
be run with `npm run test`.

I also added support for Prettier. Running `npm run format`
should run Prettier on any staged changes. I thought about adding Husky
to automatically run Prettier as a pre-commit hook, but it seems as
though Husky requires git v2.13 which would require me upgrading Git.

I didn't add support for ESLint yet since I figured that may be a
slightly more involved conversation — Prettier seems to be generally
uncontroverical and is generally only slightly configurable, whereas
ESlint is highly configurable and behaves however we want it to.

Fixing some of the 404s with SSH style repo URLs (see hound-search#19)

Adding a support section to the README