[seer]

> Parsedown is certainly very fast, but I wouldn't call it "extensible".

Parsedown is extensible and it already has been extended. There's a well working extension of Parsedown that adds support for Markdown Extra. It's called Parsedown Extra. It can be found at https://github.com/erusev/parsedown-extra

[lacksconfidence]

It is possible to extend, but extensible requires more. In this case, ParsedownExtra looks to directly extend the Parsedown class. This is fine for a single extension, but it discourages utilization of multiple independent extensions.