Add support for R / Rmd files

R / Rmd (to my knowledge) don't have simple methods for dependency management outside of installing them directly in the file you are using them in. So I guess this would involve parsing random R files searching for `install.packages` or `require` and similar statements?

There are probably more recent fancy methods of installing directly from Git repositories, but `install.packages` is the standard.

Sadly, I also don't think any version information is typically attached to those statements.