Introduce ESLint, work out rules together

[egkrly]

So the problem is that the editor loves applying it's own rules while editing, for example, " characters for automatic imports.
It also loves to play with identation and other stuff like that.

We should at least agree on these things:

• identation (I usually prefer tabs, but as I can see here you @Botffy used 2 spaces)
• is line ending semicolons needed in ts files
• typescript types - should it break the build if we don't pass at least any
• maybe function complexity? (if we really wanna torture ourselves but in a beneficial way)
• empty line on end of file

ESLint for TypeScript: https://github.com/typescript-eslint/typescript-eslint/tree/master/packages/eslint-plugin

In my opinion we should try out the default configuration first, with our own set of rules that we discuss here.