Support for Rmarkdown?

Hi, I was wondering if the plugin works with Rmarkdown? Judging by the install instructions for dein I figured it would be:

```
call dein#add('iamcco/markdown-preview.nvim', {'on_ft': ['markdown', 'pandoc.markdown', 'rmd'],
\ 'build': 'sh -c "cd app & yarn install"' })
```

I installed with vim-plug but I'm having trouble getting it to work with .Rmd files. It works with markdown files as expected.

Relevant lines from my init.vim:
```
Plug 'godlygeek/tabular' | Plug 'plasticboy/vim-markdown'
Plug 'iamcco/markdown-preview.nvim', { 'do': 'cd app & yarn install', 'for': ['markdown', 'pandoc.markdown', 'rmd'] }
```