CI: add generation of test coverage charts
As suggested in #146, we might benefit from generating our test coverage charts like this one:

![coverage_construct2](https://user-images.githubusercontent.com/157154/38455241-3b627b64-3a6d-11e8-8fe3-19726c3ef877.png)

right in the CI. Shouldn't be that hard to generate and keep them up to date automatically.