Update on test coverage

Hey,

we would like to integrate test coverage reports with Drone, but I am unable to find anything which isn’t either deprecated or not for a non-go language (https://github.com/drone/drone/issues/239#issuecomment-335409315).

We write apps in Python and would like to use e.g. something like pytest-cov, which I haven’t found any mention about. We are fine writing something on our own (e.g. having an option to parse some number from a file and publish this in some format such as {metric-value}:{failed|passed} and it would get submitted into git UI).