Feature request: support sourcepos = TRUE

[dmurdoch]

I'd like to update the RmdConcord package to support markdown::html_format (and maybe markdown::latex_format, but I'm not there yet). This requires the sourcepos = TRUE argument to be passed to commonmark::markdown_html. I can see how to do that, but the trouble is that the markdown::mark function causes problems with the result.

Specifically, with this file as input:

---
title: "Untitled"
output: 
  markdown::html_format:
    keep_md: true
---

some text

I can see commonmark::markdown_html being called twice. Once is for the main text, and a second time is for the title. When I turn on the sourcepos = TRUE option, the main text is fine.

However, the output from commonmark::markdown_html for the title is "<p data-sourcepos=\"1:1-1:8\">Untitled</p>\n". The render() function within mark() cleans this up to "<p data-sourcepos=\"1:1-1:8\">Untitled\n", and that's not valid HTML because now the tags don't balance and we have an orphan <p data-sourcepos=\"1:1-1:8\"> stuck within the title in the final output:

<title><p data-sourcepos=\"1:1-1:8\">Untitled</title>

There's a similar issue in in the body where the title is displayed.

Would you accept a PR that fixed this? I think it would be okay to completely remove the source reference when cleaning. It might be possible to fix it up to point to the YAML block, but that's probably messy.

[yihui]

Oh yes please. Thank you!