Feature request: Markdown support (UI and text/markdown from RFC 7763/7764)

Related to #1745 (and somewhat to #3171), I wish K9 Mail had an option to send and receive mails that are explicitly labeled as using Markdown (preferably in the Commonmark variant) as specified in RFC 7763 and its companion RFC 7764.

Not many mail user agents do implement these yet, unfortunately, and there would need to be some tests whether sending as text/markdown instead of rather than in addition to text/plain yields any serious compatibility problems. The expected result is that receiving MUAs just display the Markdown text verbatim.
Receiving such mails and parsing them for enriched displaying should be implemented regardless.

Expected behavior

• If K9 had a rich text editor that added formatting to the mail text #1745, there should be an option to use text/markdown instead of text/html or text/enriched (RFC 1563/1896) or text/richtext (RFC 1341/1521).
• If K9 detected that the user (supposedly) manually entered Markdown formatting, K9 should offer to send the mail as text/markdown.

Actual behavior

• K9 does not have a rich text editor.
• K9 does not detect the use of formatting characters or codes.
• K9 sends mails as text/plain (or text/html?).

Start by reviewing related issues #1745 and #3171 together with RFC 7763 and RFC 7764 to define the scope for composing and receiving Markdown mail. Done would require an agreed behavior for rich-text editing, Markdown detection, sending as text/markdown, and rendering received Markdown, along with compatibility tests.