Travis eslint prettier

[Ichag]

This PR configures eslint + prettier to lint js files.
To run it use npm run lint and for auto-fixing npm run lint-fix

[Ichag]

prettier has a strong opinion about styles, it could be that is clashes with your opinion about how the code should look:

By far the biggest reason for adopting Prettier is to stop all the on-going debates over styles. It is generally accepted that having a common style guide is valuable for a project and team but getting there is a very painful and unrewarding process. People get very emotional around particular ways of writing code and nobody likes spending time writing and receiving nits.

https://prettier.io/docs/en/why-prettier.html

[bobdenotter]

See also #113 👍

Personally, I'm fine with most conventions, as long as we pick one. Except Yoda conditions. ;-)