Add support for specifying split field

src/libexec/saturnin-dmenu

 numbered=false
 lines=10
 maybe_split=cat
+field=2-
 
 while true; do case $1 in
     -l|--lines)     lines=$2; shift 2 ;;
     --split)        delim=$2; shift 2 ;;
+    --split-field)  field=$2; shift 2 ;;
     --help|--usage) usage             ;;
     *)              break             ;;
 esac done
 }
 
 do_split() {
-    cut -d$delim -f2-
+    cut -d$delim -f$field
 }
 
 dmenu -b -i -l $lines $(style) "$@" | $maybe_split