Add alternate formatters for plain text, Markdown, etc.

[fluffy-critter]

It would be handy if there were a way to request Markdown and plain text from entry.body/entry.more/entry.footnotes.

In the case of Markdown, if the source document is Markdown then it still needs to rewrite links and images (and convert image sets into separate image tags). If the source document is HTML then it could run html2markdown on the output instead.

Plaintext can be equivalent to passing in markup=False

Potential use cases:

• Gemini protocol support I guess
• implementing this stupid thing