Fix ESLint configuration

[pascalw]

• Use babel-eslint parser for frontend code to fix ESLint parser errors.
• Lint Node.js code too.

[pascalw]

@RoelN this fixes the issue you were running into on Fraunces. While at it I noticed a few other issues with the ESLint configuration so fixed those too.

[RoelN]

Turns out my machine kept using a backed version of specimen-boilerplate-support. I had to purge it from my cache with yarn cache clean specimen-boilerplate-support.

Other than that, looking good! 👍

Edit: relevant

[pascalw]

@RoelN I've implemented the workaround suggested on the GitHub issue for that bug, which indeed seems to work. See #35.