Adding basic ESLint config

[pdehaan]

This only adds ESLint, not CSS/style linting.

Ref: #27

[ericawright]

Looks good. But prettier is already in, we don't need both, right? I haven't figured out how to customize prettier yet, so I prefer this ;)

[pdehaan]

Well... I think prettier is more for code formatting, whereas eslint is better suited for code validation and enforcing rules. Definitely some areas of overlap (indentation, quotes, etc) though.

We could use prettier as a plugin for eslint using something like eslint-plugin-prettier, but I find that flow a bit awkward and difficult to maintain.

[pdehaan]

Current ESLint stats with this PR merged:

✖ 103 problems (86 errors, 17 warnings)

Lint stats after running $ npm run lint -- --fix and auto-fixing code:

✖ 48 problems (31 errors, 17 warnings)

I can try fixing most of the issues in a follow-up PR/commit, but didn't want to cause a big diff churn here to make it easier to review.

[dannycoates]

Thanks @pdehaan!

@abhinadduri @dnarcese let's get this merged after #71 then create an issue to fix the lint errors

[pdehaan]

@dannycoates Rebased. 👀