add ext argument to md_document

[atusy]

This PR generalizes md_document by adding ext argument, so that we can generate a wide variety of formats.
Below is an example YAML frontmatter to generate pdf with ConTeXt.

---
output:
  md_document:
    variant: context
    ext: ".pdf"
---

[cderv]

@atusy does this mean for you md_document can be used with any output format ?

If I may share some thoughts about that:

It feels strange that this function designed for creating markdown output can be used for creating pdf too. From my understanding, variant for md_document seems to correspond more to Pandoc Markdown Variants and context is not one of them. It should be pdf_engine or pandoc_to to be consistent with pandoc MANUAL I guess. Using md_document as a generic output format is not the clearer. Rmarkdown works well with one function for one output type. Mixing things would mean maybe less easy to maintain in the future.

I think I would go with a more specific format with correct pandoc args, pre-processor and post-processor. It can be more suited for supporting ConText. What do you think ?

💭 Just sharing thoughts with you on this - I find it interesting when people share their concerns with me regarding my suggestions, hope it is fine with you. Cheers.

[atusy]

I'm happy to listen to your thoughts.
To be honest, I also have the strange feeling LOL.
Despite of it, I made this PR because we have bookdown::markdown_document2, which shares the context of "generalization".

Let me explain why I come up with this PR.

• Using variant for non-markdown output is sometimes useful.
  For example, variant: native renders native Haskell, and might be helpful on writing Pandoc filters.
• Generalized md_document can be a workaround to output formats that Pandoc supports, but the rmarkdown package does not support yet.

Anyway, This PR is not urging. I totally agree that one function for one output is a better desgin.

[yihui]

This PR looks simple enough, so I'll merge it. I agree it may not be an optimal solution to #1713. Thank you both!