Add support for R Markdown (Rmd) files and to ignore R package metadata (i.e. non-code files)

[hrbrmstr]

This PR adds support for R Markdown (Rmd) files. Rmd files are a mixture of document metadata, markdown/latex/html and code blocks which are fairly general purpose but are usually R or Python (similar to jupyter notebooks without the ugly JSON to deal with). The patch only counts code between the

```{lang}
```

markers.

The language type is Rmd since virtually anything could be between the code blocks, but is most likely R code.

There are also two common metadata files in R packages --- DESCRIPTION and NAMESPACE --- which have been added to the $rh_Not_Code_Filename section.

A test.Rmd and test.Rmd.yaml have been provided.

[AlDanial]

Looks good--but please also add ../tests/inputs/test.Rmd and ../tests/outputs/test.Rmd.yaml to the PR.

(also, "packages" is misspelled in 'DESCRIPTION' => 1, # R pacakges metafile)

[hrbrmstr]

ended up re-cloning the original fork and re-adding the test.Rmd[.yaml] files. No git add ops were picking them up for some reason. also fixed the spelling.

[AlDanial]

A couple of minor issues remain (test block in Unix/t/00_C.t pointing to incorrect output path and a spurious print statement in reduce_to_rmd_code_blocks()) but I'll fix them.