Add 'mark' as allowed key to i3-save-tree output

To apply this patch, use:
curl http://cr.i3wm.org/patch/700/raw.patch | git am

@@ -92,6 +92,7 @@ my %allowed_keys = map { ($_, 1) } qw(

     name

     geometry

     window_properties

+    mark

 );

 sub strip_containers {