Add JS code formatting

[baransu]

Prettier is an awesome tool for JS code formatting. We were requiring it in our README section in old UI repo.

Recently version 1.0 of prettier come out so I thought it's a good moment to add it as pre-commit hook.

I want to add similar thing for Elm later: https://github.com/avh4/elm-format

[arkgil]

Consistent code formatting is a sign of good quality 👍 Do I understand correctly that it doesn't configure actual git pre-commit hook? From what I've read some time ago, it's only possible to set them in local repositories.

[baransu]

Prettier only runs only on .js files formatting them. We're using another dependency for git pre-commit hook. In other project I had flow to typecheck JS before every commit.

[baransu]

I've updated prettier to newest version. I've also added elm-format for formatting Elm code.