allow per-milestone overriding of config settings
    
    This rejigs config parsing a bit so you can add sections to the
    config file named for fedfind milestones; if there's a section
    whose name matches the milestone of the release being checked,
    values from that section will be used with higher priority
    than values from the [main] section (but still below command-
    line argument values). This is entirely so that we can send
    Postrelease compose reports to a different list than the Rawhide
    and Branched ones, since they currently contain only cloud
    images and are only of interest to the Cloud folks.
    
    Thanks to limitations of configparser and argparse this is a
    bit icky, but not too bad.