Select2 dropdown but allow new values by user?

I want to have a dropdown with a set of values but also allow the user to "select" a new value not listed there.

I see that select2 supports this if you are using it in tags mode, but is there a way to do it without using tags?

For version 4+ check this answer above by Kevin Brown

In Select2 3.5.2 and below, you can use something like:

$(selector).select2({
  minimumInputLength:1,
  "ajax": {
    data:function (term, page) {
      return { term:term, page:page };
    },
    dataType:"json",
    quietMillis:100,
    results: function (data, page) {
      return {results: data.results};
    },
    "url": url
  },
  id: function(object) {
    return object.text;
  },
  //Allow manually entered text in drop down.
  createSearchChoice:function(term, data) {
    if ( $(data).filter( function() {
      return this.text.localeCompare(term)===0;
    }).length===0) {
      return {id:term, text:term};
    }
  },
});

(taken from an answer on the select2 mailing list, but cannot find the link now)