adding some .eslintrc files for linting

[pdehaan]

Code linting is fun!
ESLint: http://eslint.org/docs/rules/

r? @yglazko

[pdehaan]

You'll still need to add ESLint as a devDependency ($ npm i eslint -D), but we can do that later after these other PRs get merged to avoid some collisions and rebasing.

Current output:

✖ 92 problems (92 errors, 0 warnings)