move to the successor of sundown for markdown parsing, upon release

[thestinger]

Pandoc is a very large dependency since it requires building a huge tree of Haskell libraries with GHC. It would be nice to use a slimmer dependency, but Markdown is very fragmented and almost every library invents a new poorly defined variant. We are going to want some features like tables that are entirely implementation-specific. Other users of Markdown of very aware of these problems, and seem to be settling on a defacto standard implementation:

https://github.com/vmg/sundown/blob/master/CONTRIBUTING.md

[thestinger]

It doesn't look like this standardization effort went anywhere, so there's not really a point in keeping open an issue.

[lucab]

It looks like the replacement is mostly ready to go, see hoedown. They are also making tagged releases now.

Maybe this ticket can be reopened and done as part of #10103

[alexcrichton]

We talked about this in a meeting recently, and for now we're going to stick with sundown. Hoedown does certainly look promising, but we didn't want to jump on the bandwagon too soon. We wanted to wait for a more general movement of the markdown community to using hoedown instead of using sundown (which may have happened, I'm not following it very closely).

For the time being though, we've explicitly opted to say with sundown instead of using hoedown (although that's not set in stone).