Add support for specifying start_date, repeat_frequency

[deoren]

Even though this will be tricky to get right (I plan to look for existing Python libraries to handle date math), it should be infinitely more flexible than trying to shoehorn custom frequencies into the /etc/cron.d/automated_tickets file.

[deoren]

See also #7. As with that ticket, the same library used for that enhancement will probably help solve this task as well.