zoom-image plugin: Add support 'data-zoom-src' attribute of 'medium-zoom'

Enhancement request.

[medium-zoom](https://github.com/francoischalifour/medium-zoom) library can handle `data-zoom-src` attribute of images which allows to specify a HD image which will be displayed when a zoom is opened ([docs](https://github.com/francoischalifour/medium-zoom#data-zoom-src)).

Please add a some configuration option for an image in Docsify to pass the attribute to 'medium-zoom' library. It might be something like `:zoom-src`.

首先定位 Docsify 的 zoom-image 插件及其与 medium-zoom 库的集成；该 issue 未指定具体的文件或测试。跟踪图像选项如何传递给 medium-zoom，然后验证所提议的 :zoom-src 配置是否提供 data-zoom-src，并在打开 zoom 时显示高清图像。

由索引模型根据 Issue 内容生成。