Clean up / refactor the API placeholder

[nochso]

Before adding more features to it:

• Stop passing around Document and make it a property.
• Improve how Markdown is created
  • Some kind of template?

[nochso]

https://github.com/nochso/writeme/commit/0b9b6ead2a35f3fea45ff0c474b066ab22fbf8c9