How to add select years and months select Drupal 8.1 date picker?
From locating the core datepicker source code in core/modules/locale/locale.datepicker.js, I was able to inspect the $.datepicker object.

Then _defaults settings can be altered in a custom javascript fil as easy as:

(function ($) {
    Drupal.behaviors.alterDatePicker = {
        attach: function (context, settings) {
            $.datepicker._defaults.changeMonth = true;
            $.datepicker._defaults.changeYear = true;
        }
    };
})(jQuery);


All datepicker settings are documented here: http://api.jqueryui.com/datepicker/#option-changeMonth