让selectize.js显示optgroup标头

<link rel="stylesheet" type="text/css" href="path/to/selectize.css">
<script src="path/to/selectize.js"></script>

<select id="mySelect"></select>

$(document).ready(function() {
  $('#mySelect').selectize({
    options: [
      {value: 'option1', label: 'Option 1', group: 'Group 1'},
      {value: 'option2', label: 'Option 2', group: 'Group 1'},
      {value: 'option3', label: 'Option 3', group: 'Group 2'},
      {value: 'option4', label: 'Option 4', group: 'Group 2'}
    ],
    optgroups: [
      {value: 'Group 1', label: 'Group 1'},
      {value: 'Group 2', label: 'Group 2'}
    ],
    optgroupField: 'group',
    optgroupLabelField: 'label',
    optgroupValueField: 'value',
    optgroupOrder: ['Group 1', 'Group 2']
  });
});