As someone who knows markdown, I would like underscore to format my text

**Is your feature request related to a problem? Please describe.**
I see way too often that _foo_ is rendered as \_foo_ and not with italics.

**Describe the solution you'd like**
I would like to revert the change we did years ago, when we decided that rendering \_\_END__ for perl was more important than rendering it with em+strong. I think it's ok now, since the new text renderer for Convos will not render markdown syntax inside code `__END__` (backticks).

Thoughts?

Start by locating the new text renderer for Convos and its Markdown handling, then compare the current underscore behavior with the requested em+strong behavior and the Perl __END__ case. Done means underscore-delimited text renders as emphasis without regressing code-span handling.

Written by the indexing model from the issue text.