I've found this post Jekyll, modify the way some html tags are rendered
Which does explain how to extend redcarpet itself. But what else needs to be done in order to get such extension to work? What name does the extension correspond to in the _config.yml ? Do I need to include anything in my gem file?
If I simply change "markdown: redcarpet" to "markdown: RedcarpetMod" github pages will send a main telling me that there were syntax errors in the first markdown file it tries to process.