Add tests

[floledermann]

Need to investigate how to best test browser & D3 based stuff. Most important for now would be some kind of integration testing to make sure the build doesn't break on some fundamental level like in #1.

Right now there is the mapmap-examples repo that can be used to do manual testing of mapmap.js builds.

If anyone with experience in testing browser-based libraries could contribute to an automated testing setup, that would be great.

[floledermann]

We should probably use tape for this https://medium.com/javascript-scene/why-i-use-tape-instead-of-mocha-so-should-you-6aa105d8eaf4