allow for cov_source=None

None is a legitimate and useful value of cov_source, which means use the default value of coveragepy.
This results in measuring coverage for everything or (more importantly) the configured run.source in coveragerc.

This supersedes #5  (I changed the branch name, for my own sanity)
I believe I tried that and there was some issue... but I can't remember what it was :(

I'll go try it again.