Added support for setting prefilled with a function

I needed a way to fill the tags using a function, so I added it:
```javascript
    $(".tagManager").tagsManager({
        prefilled: function(){
            tags = $(".tagManager").val()
            $(".tagManager").val('')

            return tags
        },
        preventSubmitOnEnter: true,
        typeahead: true,
        typeaheadAjaxSource: '/tag/index/format/json',
        typeaheadAjaxPolling: true,
    })
```

Hope it's useful :)