Add option to be able to select all options and not just visible

I need a select list with all options selected by default. Iterating through the options as shown in the example will not select the ```includeSelectAllOption``` which I need for my select list.

This will make it possible to have both visible and not visible options selected. And it will also include the select all option.

```javascript
$("#multiselect").multiselect('selectAll', false);
 ```