Allow limiting maximum amount of dices

[thedava]

Please add an option to limit the maximum amount of dices.

Something like

--limit-dices=<num>

# Hell breaks loose
./roll 100000000d20

# Should fail with a proper error
./roll 100000000d20 --limit-dices=20

The tool allows to define the amount of dices thrown via arithmetic operations. This makes it quite complicated to verify and validate the input. I want to use this tool in a bot and need some way to limit the maximum amount of dices so the users won't kill my server