Add xclip support

Fixes #1. As first suggested in a fork by @milouse.

clipmenu

@@ -22,5 +22,9 @@ chosen_line=$(printf '%s\n' "${!selections[@]}" | dmenu -l "$dmenu_lines" "$@")
 [[ $chosen_line ]] || exit 1
 
 for selection in clipboard primary; do
+    if type -p xsel >/dev/null 2>&1; then
         xsel --"$selection" < "${selections[$chosen_line]}"
+    else
+        xclip -sel "$selection" < "${selections[$chosen_line]}"
+    fi
 done

clipmenud

@@ -7,7 +7,12 @@ declare -A last_data
 
 while sleep 1; do
     for selection in clipboard primary; do
+        if type -p xsel >/dev/null 2>&1; then
             data=$(xsel --"$selection"; printf x)
+        else
+            data=$(xclip -o -sel "$selection"; printf x)
+        fi
+
         data=${data%x}
 
         [[ $data ]] || continue