增强型下拉选择框，支持搜索与远程数据。
引用代码
<link rel="stylesheet" href="https://cdn.bootcdn.net/ajax/libs/select2/4.0.13/css/select2.min.css">\n<script src="https://cdn.bootcdn.net/ajax/libs/select2/4.0.13/js/select2.min.js"></script>资源地址
https://cdn.bootcdn.net/ajax/libs/select2/4.0.13/js/select2.min.js