Prettier support 💄

[pirelenito]

Changes the linter configuration to support Prettier and adds another npm script to format the code.

Closes #336

[pirelenito]

I've just adapted the ESLint configuration to support Prettier. Next step is attempting to run the formater.

• https://github.com/prettier/prettier-eslint
• https://github.com/prettier/prettier-eslint-cli

I've also found solutions that does the formatting automatically, but I'm not sure that is a good idea: https://github.com/danielterwiel/prettier-eslint-webpack-plugin/blob/master/package.json

It feels to me that it might make sense to have a prettier-eslint-loader that works in the same way as the eslint-loader but checks for any formatting issue... letting the user know how to fix it.

[pirelenito]

I'll merge the linting rules, and we can work in the formatting as a separated PR.