Expose SLIDE_JUMP in filelist as a configurable option

[dragulescu]

Allow the user to set the jump step in a filelist, while in slideshow mode:

$feh --slide-jump 10 -Sfilename folder/

SLIDE_JUMP_FWD, SLIDE_JUMP_BACK will use this number to jump back and forward.

This is very useful when doing a slideshow of a large filelist, when a user might want to control the granularity of the jump.

[bytebraid]

I love feh and this is one improvement I've implemented locally, adding an option for

$ feh --jump-interval 35 ....

Patch attached interval.patch.txt

I actually find both types of jumping around useful (percentage and fixed interval) which is why I chose to make this a separate option (using 'j' to jump the fixed interval and retaining the percentage on PgUp/PgDown)