Allow dice expression in Bonus roll

[Nuccia]

I think it should be interesting to add the possibility to replace the bonus roll formula by any dice expression we want.
Example :

I would like to use the dice expression in the Bonus section to replace the

0d6

by

(ceils(@classes.rog.levels))d6

‌

[Kim Mantas]

I think allowing some limited maths expressions in all of those number fields could enable some useful features.