Function appendScript

appendScript(scriptUrl, async?, defer?): HTMLScriptElement
添加 script 标签
Parameters
- scriptUrl: string
  脚本地址
- async: boolean = false
  是否异步
- defer: boolean = false
  是否延迟加载
Returns HTMLScriptElement
Example
```
appendScript('./test.js'); /// 添加一个 script 标签
```