title: Putting line number for R code with knitr
body: <p>I wonder if there is any function to put <code>line numbers</code> with <code>knitr</code> in <code>.Rnw</code>. I found <a href="https://github.com/yihui/knitr/issues/31" rel="nofollow noreferrer">this discussion</a> and some documents (now removed from the web) but could not find the way to put line numbers.</p>