Set up code linting

[kumar303]

Set up our common linting rules similar to addons-frontend.

We will be using TypeScript instead of Flow but TS linting can be done in a separate issue.

[willdurand]

Note: tslint is recommended over eslint when using TypeScript.

[willdurand]

Note: tslint is recommended over eslint when using TypeScript.

So I had lunch with a TS expert today and he told me that tslint (made by Palentir) was not super great and incompatible with ESLint. Microsoft and TS people want to improve compatibility between TS and ESLint and it seems possible to use ESLint with TS anyway, so I think we should be doing this if we can and if it is not waaaay too complicated.

[bobsilverberg]

I recall reading some things about people using ESLint even with TS, which I think is what you're suggesting above, @willdurand. If so, then 👍 to that.

[kumar303]

As discussed in #59 (comment) let's be sure to add a lint rule to enforce arrow functions everywhere.