Add Rmd R code chunck support for smart autoformatting

[inventionate]

At the moment R code chunks are unfortunately not recognized by smart autoformatting, i.e. (hA) formatting is not disabled within the code chunks (see jalvesaq/Nvim-R#600). This is due to the fact that for Rmd files the highlighting groups are overwritten by vim/nvim built-in filetype and (only) R code chunks within Rmd files are identified as rmdrChunk. I have adjusted the variable g:pandoc#formatting#smart_autoformat_blacklist in my .vimrc accordingly. However, it would probably be helpful to change that directly in the code or include it in the documentation. Anyway, thanks a lot for the great package!