The current state in master:

- markdown exporter does not extract files (leaves b64 data like HTML)
- markdown template assumes files have been extracted

The choices are:

1. make the exporter match the template (allows smaller files, markdown image links)
2. make the template match the exporter (single-file, can't use markdown images, must use inline HTML)

In both HTML and markdown, there should be a switch for whether images are embedded or not, but I am not going to make the switch in this PR, so it's just a question of which is the simplest logical default, and I went with 1. A case can be made for either one.