How to approach writing extension that slightly modifies fenced code blocks?

[racingmars]

I would like to modify fenced code blocks to allow certain text inside of them to be marked up such that it renders differently (e.g. in HTML <strong> tags). Specifically, I want to list the text of a shell/console session in a fenced code block and highlight the user-entered input text as bold.

I have looked at some of the existing extension code, but can't quite wrap my head around how to write a new Goldmark extension.

And, perhaps more specifically, I'm wondering if it is possible to write an extension that adds the capability to existing fenced code blocks (e.g. can I turn ** processing back on inside of the current fenced code blocks), or would I need to write an extension that entirely replaces fenced code blocks with a new implementation that has my additional desired behavior?

If someone points me in the right direction I can probably read more sample extension code and figure it out, but I'm not quite sure where to start approaching it.