Dice roll functions

[gergo-szabo]

Discussed in #12

[tassaron]

This looks good!

I should probably change that GitHub Action so it runs pytest before black. There is no way this would break anything so I could merge it and run black myself.

Or you can fix it yourself with pip install black; black dnd_character/dice.py

I'm busy for the next 6 hours, so I'll merge this later today. Thanks!