Add Stata & R support in fenced code blocks

[jh-min]

If opening fence is either ```s, ```{s}, ```stata or ```{stata},
fenced code block will highlight Stata syntax and support any key bindings working in source.stata.

If opening fence is either ```r or ```{r}, fenced code block will
highlight R syntax and support any key bindings working in source.r.

Also, syntax highlighting will work when adding code chunk options
such as ```{r, eval=FALSE} of R Markdown.