Adding prompt area to non-CodeCells to indent content.

This is a reponse to the problem of having really long lines in
Markdown cells, which makes the content difficult to read. Users
want wide code cells, so we don't want to narrow everything. The
solution here is to give a prompt area to the heading/md cells
to narrow their content area slightly. The only problem is that
this makes it more difficult to distinguish between output
and md content that follows that output. The solve this, we are
adding a narrow line between output and following md.