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 would suggest leaving the environment variable empty instead of using "None" as literal, this allows easier comparison:

    >>> os.environ['COV_CORE_SOURCE'] = ''
    >>> 'COV_CORE_SOURCE' in os.environ
    True
    >>> bool(os.environ['COV_CORE_SOURCE'])
    False