= Pondering ways to implement this:

== directly in cgi
https://sourceforge.net/p/roundup/code/ci/default/tree/roundup/cgi/templating.py#1460

  def hyperlinked
is used by default in the classic template to render the contents
of a message.

It would need a configuration option or another function instead of
hyperlinked.

== as extension
registering as a utility function just like
 
https://sourceforge.net/p/roundup/code/ci/default/tree/share/roundup/templates/responsive/extensions/local_replace.py
and then changeing the template like
https://sourceforge.net/p/roundup/code/ci/default/tree/share/roundup/templates/devel/html/task.item.html#228
in all relevant places.

== candidate libraries
There is https://pypi.python.org/pypi/CommonMark/0.7.5

Otherwise github-style makes sense which I guess is done best by using
https://pypi.org/project/Markdown/ and the guide to configure the extensions
https://facelessuser.github.io/pymdown-extensions/extensions/github/