Add shebang and set executable bit on ~/.fehbg

Before, feh would output this file to `~/.fehbg`:

    sircmpwn@sayaka ~/s/feh master> cat ~/.fehbg
    feh  --bg-fill '/home/sircmpwn/wallpaper.png' 
    sircmpwn@sayaka ~/s/feh master> ls -l ~/.fehbg 
    -rw-r--r-- 1 sircmpwn sircmpwn 47 Aug 21 11:22 /home/sircmpwn/.fehbg

With this patch applied, it instead does this:

    sircmpwn@sayaka ~/s/feh master> cat ~/.fehbg
    #!/bin/sh
    feh  --bg-fill '/home/sircmpwn/wallpaper.png' 
    sircmpwn@sayaka ~/s/feh master> ls -l ~/.fehbg 
    -rwxr-xr-- 1 sircmpwn sircmpwn 59 Aug 21 11:23 /home/sircmpwn/.fehbg*