It would  be nice to allow use of some markup language when creating
messages.

This allows people to set off blocks of text, add headers etc. to make
it easier to scan the message body.

Since messages have mime types, we can save the file as mime type:

text/x-rst

text/x-markdown; charset=UTF-8

etc.

Also this would be helpful in better representing html email.

See issue2550799.

Also this would be helpful with implementing:

  http://www.roundup-tracker.org/cgi-bin/moin.cgi/RoundupAsWiki