Allow for Email Digest Notifications (Summary Notifications)

[aedelbro]

I like getting email notifications when users request new entries, however if someone goes on a binge and requests 50+ items in a short amount of time, I don't want to get 50+ emails.

Optimally, I should be able to select a duration of time that it aggregates the emails (or other notification systems).

ie, I would like at most 1 email per day, with a summary of all requested items, instead of an email for each item.