Add support for Fate Dice

[piersb]

/roll dFate - roll four fate dice /roll dfate, /roll dFate, /roll df, /roll dF - alternate calls for above

/roll XdFate - roll X fate dice /roll Xdfate, /roll XdFate, /roll Xdf, /roll XdF - alternate calls for above

[piersb]

Mostly duplicates #9, only difference is the alternate calls

[arkie]

All of the second syntaxes should work as of https://github.com/arkie/hackyslack2/pull/23 (and the non-capitalized versions should have already worked) but based on https://en.wikipedia.org/wiki/Fudge_(role-playing_game_system)#Fudge_dice currently keeping the default to one die (mostly for consistency with the dice notation describing the default as one die).