Add pandoc as an engine of fmt_markdown()

[atusy]

Currently, fmt_markdown internally uses commonmark package for conversion of markdown texts.
I would like to suggest pandoc as an optional engine.
In this way, we can use Pandoc's Markdown which is consistent with R Markdown.
This will allow wider variety of inline formatting such as superscript and subscript, and also allow complex features like citations.

Just to give a hint to implement citations, ftExtra::colformat_md for flextable internally reads rmarkdown::metadata to fetch bibliographies (https://github.com/atusy/ftExtra/blob/master/R/md2ast.R#L8), and then output citation keys as a part of YAML meta data block so to invisibly make sure citations appear in the reference (https://github.com/atusy/ftExtra/blob/master/R/knit-print.R#L20-L24).

[maia-sh]

+1 for this feature. In the meanwhile, is there any existing workaround to add a citation (of the format @mycitationkey) to a gt table?

[kylebutts]

+1! It would be nice to be able to have citations within a table!

[LukasWallrich]

Until this is implemented, it would be great to document the limitations (or available MD features) in ?md - it took me a while to understand why ~sub~script worked in RMarkdown until I moved it into a table.