Add dice "reroll" (r) notation

[Vlek]

Add the ability to denote that a dice should be rerolled under certain conditions:

(Dice) R/r [number]

4d6r1 - Roll 4 d6 dice, reroll a die once if it lands on 1

4d6R1 - Roll 4 d6 dice, reroll a die every time if it lands on 1

Considerations:
1d1R1 could be an infinite loop. Must either have this as an error case or simply return 1 as the only possibility.

[Vlek]

What if we want to re-roll if it's the case that it lands on one of a couple of different numbers? We could do a list: (Dice) R/r [number] or (Dice) R/r [ (number) [, number] ]

[Vlek]

If it's the case that we get something like 1d1r where it's an infinite loop, it needs to simply return 1 in that case.