Default tokenizer. This function uses breaks the input on substring match of any string from the
opts.tokenSeparators array and uses opts.createSearchChoice to create the choice object. Both of those
two options have to be defined in order for the tokenizer to work.

@param input text user has typed so far or pasted into the search field
@param selection currently selected choices
@param selectCallback function(choice) callback tho add the choice to selection
@param opts select2's opts
@return undefined/null to leave the current input unchanged, or a string to change the input to the returned value
function defaultTokenizer(input, selection, selectCallback, opts) {
        var original = input, // store the original so we can compare and know if we need to tell the search to update its text
            dupe = false, // check for whether a token we extracted represents a duplicate selected choice
            token, // token
            index, // position at which the separator was found
            i, l, // looping variables
            separator; // the matched separator

        if (!opts.createSearchChoice || !opts.tokenSeparators || opts.tokenSeparators.length < 1) return undefined;

        while (true) {
            index = -1;

            for (i = 0, l = opts.tokenSeparators.length; i < l; i++) {
                separator = opts.tokenSeparators[i];
                index = input.indexOf(separator);
                if (index >= 0) break;
            }

            if (index < 0) break; // did not find any token separator in the input string, bail

            token = input.substring(0, index);
            input = input.substring(index + separator.length);

            if (token.length > 0) {
                token = opts.createSearchChoice.call(this, token, selection);
                if (token !== undefined && token !== null && opts.id(token) !== undefined && opts.id(token) !== null) {
                    dupe = false;
                    for (i = 0, l = selection.length; i < l; i++) {
                        if (equal(opts.id(token), opts.id(selection[i]))) {
                            dupe = true; break;
                        }
                    }

                    if (!dupe) selectCallback(token);
                }
            }
        }

        if (original!==input) return input;
    }