Configuration option for filtering created items

[nlwillia]

This is a feature request.

In create mode, it would be useful to be able to provide a predicate function as a configuration option that would take the current search value and return whether to allow an option/item to be inserted for it. Like the function variant of the current create option, the filter option function should accept a value and a callback so that the filter can be implemented synchronously or asynchronously.

For example:

$(...).selectize({
  create: true,
  createFilter: function(value, callback) {
    return ...; // some truthy result based on value OR undefined with the intent to invoke the callback later
  }
});

[nlwillia]

I realized later that an asynchronous option for the createFilter function wouldn't work with the input model, so disregard that part. In the pull request, I've included support for regexes as well.