Allow alternative ways of syntax highlighting

[Aarkon]

Hi!

First of all, thank you for your great work! 😃 I'd like to propose an enhancement, which would be a (documented) way to exchange Highlight.js with something ideally happening at build time, or at least some internal function that is called when displaying code blocks in HTML. Your official documentation says this is already possible, but not how to do it:

Code blocks have syntax highlighting via Highlight.js (but you can set up something else)

If it is true what this line says (I have no doubt it is 😉), this should merely be a writing task, with no coding at all.

elm-pages for instance has syntax highlighting without third party runtime libraries. Their solution looks like hooking into markdown rendering itself though, which I as an elm beginner don't dare to touch already. Don't get me wrong, I don't want to spark any jealousy here - elm-apges doesn't work for me for some reasons, but maybe there is some inspiration to be taken from this project in this regard.