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

[mnasyrov]

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.

[trusktr]

Would the syntax be [link](/demo.md ':data-zoom-src=image-hd.jpg')? Note, I used data-zoom-src so that it is clearly mapping to the data-zoom-src attribute.

I think maybe we can also think about how to more generically pass any attributes. F.e. foo-bar or data-foo-bar, data-lorem-ipsum, etc, if we're not already doing that.