use pandoc as backend

[lorenzwalthert]

Seem like RStudio visual mode uses something like this to convert .Rmd into standard pandoc markdown:

pandoc -s -f markdown+backtick_code_blocks+fenced_code_attributes -t markdown -o new-README.Rmd README.Rmd

with code chunks:

``` {#r eval=FALSE}
a::b()
1
```

Maybe they could even expose the functionality. Also see: rstudio/rstudio#7922

If they don't want:

• adapt header to required format above, i.e. prefix langauge with #, remove all commas (with regex).
• render.
• go through all headers and replace them with the old version.

Would be a nice pre-commit hook.

[lorenzwalthert]

Reference: r-lib/styler#941