move offset to outer loop

I could be wrong (I'm not much of a dnd guy), but my friends (who are dnd guys) noticed that the offset was being added/subtracted to every roll rather than to the sum of the rolls. This pull should fix it according to https://en.wikipedia.org/wiki/Dice_notation

To see the symptom in action one can tell the bot to "!roll 100d1-1" which will return 0. With this pull request it returns 99 (and "!roll 2#100d1-1" returns 198).

Let me know if this is wrong or if there is anything I've overlooked. Thanks!