Write display() calls in markdown output

[grantbruer]

@fredrikekre I made some changes to get the display() calls rendered for the markdown output, but I don't understand the multiple MIMEs for the same display() call. And I don't understand the use for the split_mime() call.

So this is an initial fix, but it seems to duplicate output to both plain text and HTML. Can you advise on the correct behavior?

Fixes #255