Add a rivets_index data value that mirrors ${object_name}_index; this

allows for general-purpose functions to always look at the former.  It
makes data-class-even="workers:isEven" possible.

@@ -399,7 +399,7 @@ Rivets.binders =
           data = {}
           data[n] = m for n, m of @view.models
           data[@args[0]] = item
-          data["#{@args[0]}_index"] = i
+          data["#{@args[0]}_index"] = data['rivets_index'] = i
           itemEl = el.cloneNode true
           if iterated.length > 0
             previous = iterated[iterated.length - 1].els[0]