TODO:
- cleanup asserts into proper exceptions

Nice to have, but unlikely to be implemented:
- more than just markdown (restructured text, html, etc)
- configurable banner at the top of rendered pages ("this uploaded from...")
- generate disclaimer header / banner with link to canonical source
  - rather than raw HTML that triggers WARNING: Undefined function 'RawInline'
- color output, one line per file
- detecting and uploading included image files as attachments.
  `confluence-sync` does this