Example of an html document with syntax highlighting not supported natively by R Markdown (SAS & Crystal).
For a full description of how this is accomplished and stitched together, refer to my blog post.
For future reference, this makes use of a super simple post-commit hook for git that copies the html document into the docs folder so that it can be viewed on Github Pages — https://curtisalexander.github.io/rmarkdown-with-alt-langs/.
Create the file .git/hooks/post-commit with the following.
#!/bin/sh
cp rmarkdown-with-alt-langs.html docs/index.html
git add docs/index.html
git commit -m "$(git log -1 master --pretty=%B)"