use html comments to add extensions

[legrostdg]

#2069 was closed, but I think that @lierdakil had a great idea: use html comments to add extensions. Something like this:

<!-- pandoc-markdown: +compact_definition_lists -->

That would be a nice feature to have in pandoc.

[mb21]

I guess this falls under #4627 ?

[mb21]

E.g. with panrun you can do panrun input.md using:

---
output:
  html:
    from: 'markdown+compact_definition_lists'
    to: html
---

Term 1
  ~ Definition 1
Term 2
  ~ Definition 2a
  ~ Definition 2b

[jgm]

If we did this at all, I think it would make more
sense to have a YAML metadata field:

---
extensions: +compact_definition_lists -pipe_tables
...

One possible problem with this is that parsing YAML
metadata is itself an extension. So is parsing raw
HTML (including comments).

We'd also have to change the parser to keep extensions
in state, and possibly modify them on the fly, rather
than getting them from reader options.

[mb21]

Closing in favour of #4627