Reconsider using rmarkdown::md_document("gfm") or rmarkdown::github_document()

[jennybc]

This came up recently in #375 and #383 where @cderv points out that reprex would not benefit from a fix for this in rmarkdown itself, because reprex_document() isn't making direct use of rmarkdown's explicit functionality for preparing markdown for GitHub.

Historically, I know I had a good reason for this, however it's not at all clear if that still holds. Vaguely, it feels like rmarkdown went through a long phase where github_document() was not, actually, the best / most modern way to prepare markdown for GitHub. I think that's why I started using md_document() in the first place here in reprex. But the world has changed since then.

Philosophically, I'd love reprex to have ZERO knowledge of Pandoc minutiae and leave that for those maintaining rmarkdown. That is why I don't love this fix 293225e, but I'm taking it for now.