Support for GitHub Flavored Markdown Alerts and Notes

[lschierer]

The POD documentation simply states that 'some of the Markdown extensions' are supported, but not which. From my own use, it appears that Alerts aren't one of the supported subset. Please consider adding support for this extension to the standard markdown syntax.

Thanks!!

(as an aside note, I just found the Markdent set of modules, and I love that I can use it to insert classes for styling the html produced as I parse my markdown, so that I end up with something that shows consistently with the rest of the site I'm building!)

[autarch]

I'm not really up for adding new features to my Perl modules these days (see https://blog.urth.org/2023/02/11/all-my-perl-modules-are-in-maintenance-mode/). But if you want to submit a PR I can review it and do a new release.