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`.