add option to directly manipulate (reduce) a dm reward pool

[7uci3n]

As a DM, when I am drunk and don't type no good, I want to be able to reduce (maybe not even increase) my dm reward pool manually.
This might be appropriate if I needed to retype a reward for a player due to a mistake which in turn might have given me more dm rewards than appropriate (double dipping, basically).

[a-h-i]

The idea was to enable you to basically reward with negative numbers so that you can undo the whole process if you made a mistake. However this was never properly tested on the testing server so it was left out of the wiki documentation. Further testing should be done and laid out for v1.1