rollstats: add more modes

[Zadest]

• add !rollstats -min to reroll untill no sum is below 8
• add !rollstats -min=X to reroll untill no sum is below X
• add !rollstats -higher=X to reroll untill total of all sums is bigger than X
• add !rollstats -max=X to set one of the rolled dice per stat to the dice maximum value