option to move `~/.fehbg`

[0x08088405]

Hello, would it be possible to add a feature to specify where the .fehbg file goes? Looks like it's hardcoded here...

https://github.com/derf/feh/blob/9642354668f0555f125a6684a63d7ec30c427960/src/wallpaper.c#L271

I see that there's --no-fehbg to disable it, but I do want it, it's a good feature, just not in the root of my home directory. Seems like the rest of feh respects the XDG base directory specification, maybe this could too? Keeping in mind to not break anyone's existing xinitrc, of course.

Thanks in advance!

[avlec]

Environment variable would be desirable as there are concerns about increasing the number of command line options. The only other idea I had regarding this would be to change --no-fehbg to something like --bg-loc and set the value to /dev/null for the old behaviour.