Fix Markdown usage to work with Markdown 3.0+
Markdown 3.0 and later no longer considers the emphasis modifiers
to be an extension, and has them always enabled. We still enable
the 'smart_strong' extension when we detect older versions of
Markdown to retain backward compatibility.
Signed-off-by: Neal Gompa <ngompa13@gmail.com></ngompa13@gmail.com>