Add cm_item_list element type which supports render arrays as items

Sadly theme_item_list does not support render array elements as the values and must be passed them as strings.

This is sad for those of us who want to return render arrays allowing later peeps to come along and inspect / alter their output.

Solution! Define a new '#type' => 'cm_item_list' which simple calls through to theme('item_list') but not before a #pre_render callback renders all items which are determined to be render arrays.

Nice.