Images (and generic Webpack assets)

[benglynn]

Allow markdown to reference images. Use scan to create variants with a site-wide spec.

This is likely to break down into three related tasks:

image dependencies

A page-pipe pipeable. Glean from markdown metadata and content. Changes to any image source then trigger a render.

Image versions

• Part of the render pipeline as we don't want to do this work unnecessarily. Generate Buffer for each image variant.
• This is destined to be emitted much like html, perhaps a generic assets would mean tapAsyncCallback is better encapsulated?
• Related to the previous point, why is Webpack 'hidden' from CompileContext? Perhaps address that here, the webackReducer can always be stubbed in tests

figure

Client code to enhance the low-res variant in source to a responsive image.