Add eslint and stylelint configs / script

[coliff]

I think it'd be good to add a .stylelintrc and a eslintrc.json to the projects root and add lint-js and lint-css tasks to package.json. We can use the recommended/standard configs for both. what do you think? I'd be happy to add a PR for this...

[paulirish]

i've been using prettier on this project so far and liking it. let's just do that. i don't really want to block any PRs on basic linting stuff here. the codebase is too small to really go too hard. :)

for prettier i guess we just need a config file and an npm script. don't wanna do any hooks or w/e.

[coliff]

Great. I’m a fan of Prettier too, I’ll open a PR later today.