add support to Fate / Fudge dice

[OliverLSanz]

Hi! Nice project, good work :-)

I am using py-rolldice in the online text game I'm developing.

But I'll love to be able to roll fate dice as included in the CritDice syntax.

They are simply d6 dice with two -1 results, two 0 results and two +1 results, and their syntax is "dF". They effectively are d3 with -1, 0 and 1 as faces.

Thank you for your time!