Weekdays and days are only additive if both specified

Closes https://github.com/pbyrne/cron-checker/issues/10

I had misunderstood the docs on this feature, but I think I understand now.

* If both weekday and day are *, run every day
* If both weekday and day are specify, run on either match
* If day is specified and weekday is *, run when day matches
* If weekday is specified and day is *, run when weekday matches