[dropdown] actionable item + callback

[jamessampford]

Allow a dropdown to have an actionable item, this is, doesn't get chosen as the value - by this, imagine having a dropdown where at first there is no data, however you could append an actionable item in the list, which when selected could launch a model to create a new record and the developer could add it to the dropdown before/after the actionable item as the selected item

For example, the actionable item is called "+New"
<div class="item actionable" data-value="new">+New</div>
Clicking/Picking this item fires off a callback, possibly with the data-value and text of the item (dropdown value+text remains at existing value)

$('.ui.dropdown').dropdown({
    onActionable: function(value, text, $selected){
        ....
    }
});