Enforce code linting

[anibalsolon]

Use a code linting tool, such as ESLint, to keep the code organized and easier to navigate. It can be added to the CircleCI workflow, to auto-lint and display problems that it cannot solve by itself (blocking a PR to be merged until fixed).

[anibalsolon]

I've started linting the project, but there are some parts that require manual intervention (meaning prone to human error). What you think would be the best way to do it? @katjaq @r03ert0 first a PR with what ESLint could fix, and then another with the manual ones?