Inclusion of parent comments in notifications

Currently, outgoing messages include only a copy of the comment that was just posted. However, on traditional mailing lists each message includes a copy of all of the parent messages before it (leading to the heated and infamous "reply-above" vs "reply-below" debate). This can help to provide context on mail clients that don't support threading. Thus, it might be helpful to have a per-user-configurable option to include copies of a certain number of parent comments in outgoing messages.

This idea came from (and should probably wait on) #1054334: Convert message cleaner and signature separator to input filters- I just want to post it while it's fresh in my head.