add backlight script and binds

.config/sxhkd/sxhkdrc

@@ -138,6 +138,14 @@ super + {Left,Down,Up,Right}
 super + shift + g
     $SCRIPTS/toggle-gaps
 
+#
+# screen brightness
+#
+XF86MonBrightnessUp
+    $SCRIPTS/backlight +
+
+XF86MonBrightnessDown
+    $SCRIPTS/backlight -
 
 #
 # media keys

.scripts/backlight

@@ -0,0 +1,22 @@
+#!/usr/bin/env bash
+
+step=2
+value=`xbacklight -get`
+
+# double, halve value or bail
+if [ "$1" == "+" ]; then
+    let value*=$step
+elif [ "$1" == "-" ]; then
+    let value/=$step
+else 
+    echo "usage: backlight +, backlight -"
+    exit 1
+fi
+
+# check range
+min=1
+max=100
+[[ $value -lt $min ]] && value=$min
+[[ $value -gt $max ]] && value=$max
+
+xbacklight -set $value -steps 10