cron/senddigests needs an exceptlist option

[Mark Sapiro]

Sites sometimes want to send digests on a different schedule for one or a few lists. This is currently difficult as it requires running cron/senddigests with, e.g., '-l listname' or '-l list1 -l list2' for the one or two and separately with '-l listn' for all the others, and this needs to be updated as lists are added or removed.

With a -e/--exceptlist option, only two crons are required, one with, e.g., '-l list1 -l list2' and one with '-e list1 -e list2'