Enable coveragerc

The essential idea is that there should be a way to enable pytest-cov without overriding the source setting in .coveragerc.

The new test requires the pull request to cov-core here: https://github.com/schlamar/cov-core/pull/5

This is my second implementation. You can compare to the first here:
Find the diff here: https://github.com/bukzor/pytest-cov/commit/ebfcf418ec776b3fff165be92c9fb9d674790593