Add more tests

Couldn't find an issue directly addressing this, but it would be pretty nice, if not essential, that there's a good testing framework for making sure that changes do not regress the code. I understand that while the code is still in flux testing does not matter that much, but given that there is already quite a rich and working feature set, it'd be a shame if it became buggy.

Personally, I think Jest is the best testing framework in this context because of its integration with React components. Have to admit I haven't used it myself, though.