select all/clear all

That would need javascript, so would not be straightforward, though possible.

Including an "all" option would ideally need some javascript to do the following:

* Set checked/unchecked all other checkboxes when "all" is checked/unchecked by the user.
* Set the "all" checkbox as checked when user manually checks all other checkboxes
* Clear the "all" checkbox "all" was selected and the user manually unchecks one of the other checkboxes.

Without this functionality, to make removal of "all" a configurable option should be fairly easy. I'll try it.