New abbr syntax

I can see the benefits of the new `[>ABBR]:` format and its inline variation, but is it possible that MMD 6 could still render the `*[ABBR]:` format as well? This is one of those syntax changes that's going to break older documents, especially because other variants support it as well (PHP Markdown Extra, Maruku, MMD 5, kramdown), and anyone who uses Markdown on a blog is going to have "legacy" syntax...