Ignores size attributes / doesn't support 2x Retina images

[peterflynn]

I'm trying to include diagrams in my documentation that look crisp on high-DPI screens. This requires setting their display size to 1/2 their actual pixel dimensions -- otherwise, the images will look huge and still be fuzzy on 2x screens.

The Fancybox plugin is posing several problems for this though:

• Ideally it would have some nice behavior to set up the right max size for @2x images automatically, but it does not.
• I can't manually set the right size using an <img> tag with width/height attributes or a style attribute, since this plugin replaces the entire tag with a template that blows away all attributes on the tag (see https://github.com/ly-tools/gitbook-plugin-fancybox/blob/master/index.js#L8-L10).
• I can't find any way to make the plugin ignore certain <img> tags either.