Add Prettier, set up CI

[psvenk]

Fixes #54. ESLint is already configured with create-react-app, so this just adds a script for running Prettier and sets up CI for Prettier, ESLint, and Black.

[cjquines]

for black, we either have to add instructions on how to run black, or maintainers need to commit to running it against PRs when someone submits something unformatted