update r.snippets and add rmd.snippets for rmarkdown (copied from https://github.com/jalvesaq/Nvim-R/tree/master/R)
update r.snippets and add rmd.snippets for rmarkdown (copied from https://github.com/jalvesaq/Nvim-R/tree/master/R)

needle-wang/vim-snippets,kevinkirkup/vim-snippets,phatograph/vim-snippets,vito-c/vim-snippets,adam-e-trepanier/vim-snippets,phatograph/vim-snippets,adam-e-trepanier/vim-snippets,needle-wang/vim-snippets,craigp/vim-snippets,craigp/vim-snippets,kevinkirkup/vim-snippets,honza/vim-snippets,honza/vim-snippets,vito-c/vim-snippets