Add backlight control

dotfiles/i3/config

 bindsym $mod+Shift+c        exec "saturnin clip load"
 bindsym $mod+Shift+f        exec thunar
 bindsym $mod+Shift+o        exec "urxvt -e mocp"
+bindsym $mod+d              exec xbacklight -dec 50
+bindsym $mod+Shift+d        exec xbacklight -inc 10
 
 bindsym $mod+Escape         exec "saturnin iam afk"
 bindsym $mod+Shift+Escape   exec "saturnin iam gone"