+) Allow to set select2.minimumInputLength from AjaxSelect2Widget. Default to 1

I ran into an issue where I needed to set **select2.minimumInputLength** to 0 for **ModelView.form_ajax_refs** to show some input when the user hasn't entered a character on the field yet. Here I propose a generic behavior where the programmer can specify the requested **minimumInputLength** with a default value of being 1, so the fields will continue to work as they have been working if **field.minimum_input_length** is not set explicity. Please, consider adding this change.