Add scratchpad bindings

dotfiles/i3/config

 bindsym $mod+Shift+9 move container to workspace $w9
 bindsym $mod+Shift+0 move container to workspace $w0
 
+bindsym $mod+Shift+minus move scratchpad
+bindsym $mod+minus scratchpad show
+
 # reload the configuration file
 bindsym $mod+Shift+c reload
 # restart i3 inplace (preserves your layout/session, can be used to upgrade i3)