Supply source function as string with data attribute

Created by: koenpunt

Allow data attribute data-source to be a reference to a function.

e.g.:

<script>
var typeahead_source = function(query, process){
    return ["Alabama","Alaska","Arizona","Arkansas","California","..."];
}
</script>
<input type="text" data-provide="typeahead" data-source="typeahead_source">