Fisherman's Gambit currently mishaps for negative indices. This change would instead make it invert its operation, eg. where 3 fisherman turns x y z into y z x, -3 fisherman would turn x y z into z x y.

I'd be happy to put together a PR for this.

I'm going to fold this issue into #291 and track progress there, but by all means please, if you fix up the PR i'll pull it