Use coveralls.io to display testing coverage?

Created by: ollieparanoid

PR #760 will integrate automatic testing on Travis CI, at least for the fast testcases. I would like to display how much testing coverage we have for pmbootstrap in the README.md (with a badge and with details when you click on it).

Proprietary service coveralls.io looks like a nice interface and is free to use for open source projects. It is compatible to GitHub and gitlab in case we decide to move there.

I have not found a similar service, where the source code is available (is there any?).

Worst case is, that for some reason we suddenly can't use that service anymore and lose the coverage reports from the past. We can always generate coverage reports ourselves with pytest-cov (which is the output that gets used for coveralls.)

And of course the upside is, that this motivates (at least me) to make more testcases and have good testing coverage.

So what do you think about this proposal, fellow hackers?