This pull request adds a new pattern for defining R code chunks within a LaTeX environment, replacing the <<>>= ... @ syntax.

\begin{Rcode}[options]
## code goes here
\end{Rcode}

The pattern is associated with files with an .RLatex extension. Ideally, I would like this syntax to be just another option for .Rnw files, but for now, I think this approach is less likely to create conflicts with existing code.

Why did I do this? My development environment is emacs/ESS, and when writing ordinary LaTeX documents, I use the preview-latex emacs package to preview LaTeX math inline. However, preview-latex chokes on R code chunks, and I have not found a way to tell it to skip everything between <<>>= and @. But I can tell preview-latex to skip an environment. So preview-latex thinks the code chunk is just an environment to skip, and knitr knows that the Rcode environment contains code to be knitted. Everyone is happy.

I created a gist that goes into more detail, with examples (including some additions to the LaTeX preamble). The gist also contains a new polymode, so ESS can recognize the contents of the Rcode environment as code, and execute syntax highlighting, indentation, etc., appropriately. That's not part of knitr, but I mention it here for those who are interested.

Of course, I could just define the pattern in a hook to run whenever knitr loads. But that would make my documents non-portable. So I am hoping this contribution is worthy of addition to the package, and that others can benefit.