Support --embed-resources in Markdown output formats

[brunnerh]

Describe your proposed improvement and the problem it solves.

Images (and potentially other resouces) could also be embedded via data: URLs.

The URLs can be a bit disruptive in the middle of the document, so for images they could also be moved to the end by using named references, e.g.

Implement as seen in this mockup:
![application screenshot][img1]

...

[img1]: data:image/png;base64,...

Describe alternatives you've considered.

Well, one alternative that I use right now is running pandoc twice:
Source => HTML with --embed-resources => Markdown