Sortable: Document and support the object version of the placeholder option

I have used the following hack in order to customise the placeholder that Sortable uses:

$(selector).sortable({
  placeholder: {
    element: function(currentItem) {
    },
    update: function(container,p) {
    }
  }
});

This hack works in all versions of UI since 1.7.2 - the one I'm using. I got the hack from StackOverflow here:

http://stackoverflow.com/questions/2150002/jquery-ui-sortable-how-can-i-change-the-appearance-of-the-placeholder-object

This is actually extremely useful in a variety of situations and I think it should be officially part of the API. I would suggest tweaks to this: the currentItem, for example, is not the ui.item we get in other functions. So, a bit of tweaking of the interface would be needed, but this is so amazingly useful I think it would be worth it.