Add a templating function for option list

Add a ```renderItem(item)``` function to enable custom templating. Use by passing the function as an option when initialising the dropdown:

```
$('.dropdown').easyDropDown({
   renderItem: function(item) {
       var $item = $(item.domNode);
       return '<span class="item">' + item.title + '<span>' + $item.data('attribute') + '</span>'</span>'
   }
});
```