automatically handle special case if user maps SneakFoo to f

[justinmk]

if user maps sneak to f, don't make them let g:sneak#nextprev_f = 0

[reicheltd]

Hi,

Ive mapped to f but in the doc it says let g:sneak#nextprev_f = 0 but good point!

[justinmk]

Edited, thanks.

[justinmk]

@reicheltd If you mapped f to Sneak, you can remove the nextprev_f option from your vimrc now.

Incidentally, I also renamed the option to f_reset (and t_reset), which is slightly less horrible than the old name.