Calculate code coverage metrics during CI

[schornakj]

Right now Yak has a code coverage metric of zero percent (profoundly shameful!). I think that seeing a reminder of that for every PR would be a good incentive to make that number something other than zero.

Tesseract added Codecov integration recently, and I like the reports it generates (an example here). Since Yak is an open-source package Codecov is probably a good option here as well.