Allow hidden items

Allow the option of have hidden items in the elements array. In my case i' using the multiselect for build a selector thats allow hide/show columns in a table/grid, the array with the columns definition have extra data needed by the gridplugin, with this extra parameter i can keep that extra data in the array without need to pass a filtered array to the multiselect and re-build the original array using the output-model data.

I can think a few more case of use for this feature if needed.