Modernize code to ES6+

[jankapunkt]

I think we have a huge potential to transform as many variables as possible to const and let which allows us to have better scoping (block-scoped variables) and let's us detect unintended mutations early.

We already replaced many lodash functions with native ES6 alternatives so I think it might be good to consequentially do this for the rest of the code, too.

This issue is weakly related to #48

[HappyZombies]

@jankapunkt is there any other place we need to update for ES6+? If not we can close this item :)