format code with Prettier

[boneskull]

This adds Prettier w/ ESLint integration. Formatting is managed via prettier-eslint-cli which formats with Prettier then applies ESLint fixes thereafter.

The only noticeable conflict with our semistandard-based ESLint config is the lack of a space after the function keyword; this particular rule is suppressed by eslint-plugin-prettier and its ilk.

The motivations here:

1. Consistency
2. Automation
3. Avoid debate about code style

lint-staged and husky are here to automatically format staged JS files as a pre-commit hook. I personally use this setup in other places and it hasn't caused any friction.

[coveralls]

Coverage remained the same at 90.009% when pulling 5759baf on prettier into efff26f on master.

[Bamieh]

a huge +1 on this. I personally prefer the functions to be written like this.

[outsideris]

Automation is always good!

[Munter]

Awesome!