It should be really easy to port, I'll see if I can do it.
In the meantime, if you're familiar with TamperMonkey (allows you to inject javascript in websites) you can visit Showdown, add a new script and copy/paste this one:
https://gist.github.com/DidItTeraDev/5b3530c7d1b0f59213dd1aaa647097ef...