Implement ESLint on the repo

[DamonOehlman]

Given there's a few of us committing code to the repo now, all with slight variations in style I think we should probably introduce eslint (and probably an .editorconfig file) to help us keep things consistent. I'm pretty flexible with code style, so @5punk if you have strong opinions LMK. If not, perhaps we should just adopt one of the well known eslint styles such as eslint-config-airbnb so the barrier to entry to contributing doesn't feel too high.

[5punk]

Prettier