code: function loadScript(url, done) {
    var script = doc.createElement('script');
    script.src = url;
    script.onload = onreadystatechange = function() {
      if (!this.readyState ||
          this.readyState === 'load' ||
          this.readyState === 'complete') {
        done && done();
        script.onload = onreadystatechange
        script.parentNode.removeChild(script);
      }
    };
    body.appendChild(script);
  }
docs: 动态加载外部脚本
  @param  {String}   url [脚本地址]
  @param  {Function} done  [脚本完毕回调函数]