TODO:
    run other targets even if one target fails due to a error (but fail on bugs...) (base class Exception?)

0.3.0:
    pvl.backup.mount: mount without readonly fails to omit --options arg
    XXX: rsync-snapshot clean_interval removes newest items
    rsync-wrapper: uses /mnt
    better config error when referencing an undefined interval for [target:$:intervals]

0.4.0:
    XXX: bad implementation
        different run-intervals?
        bi-daily intervals?

0.5.0:
    config:     split pvl-backup.d configs
    logging:    less config/source-parsing, more 'real rsync command running'