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

Enhancement request.

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).

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.