Add Roll History

[starcraftman]

!reroll -> Roll last roll again.
!reroll --choose -> Present scrolling list of last N rolls.
!reroll -2 -> Roll 2 rolls ago.

Keep history pruned, sequential dupe roll strings should not be stored.

Let N be configurable in config, persist history in DB. Prune on addition.

[starcraftman]

Been merged a short while.