title: knit_child in a Rmd file is printing unwanted output
body: <p>I had succesfully used <code>knit_child</code> for generating pdf files, following the code of <a href="http://yihui.name/knitr/demo/child/" rel="noreferrer">http://yihui.name/knitr/demo/child/</a>, but when I try to use that example in a <code>.Rmd</code> file: </p>
  
  <pre><code>```{r, results='asis', echo=FALSE, message=FALSE}
  out = NULL
  for (p in c("p1","p2","p3","p4","p5","p6","p7","p8","p9","p10")) {
    out = c(out, knit_child('quick_variable.Rmd'))
    cat(out)
  }
  ```
  </code></pre>
  
  <p>(I modify the original code, for work in <code>Rmd</code>).</p>
  
  <p>I have two problems, the first one:</p>
  
  <pre><code>|
  | | 0% |
  |... | 5% ordinary text without R code
  
  |
  |....... | 11% label: unnamed-chunk-4 (with options) List of 1 $ echo: logi FALSE
  
  |
  |.......... | 16% ordinary text without R code
  
  |
  |.............. | 21% label: unnamed-chunk-5 (with options) List of 2 $ echo : logi FALSE $ results: chr "asis"
  .... 
  (the output follows)
  </code></pre>
  
  <p>Obviously all this output is unwanted. I believe that this problem is related to the use of  <code>cat</code> in the code above, but if I remove that, no output, no plots is printed. What can I do for solving this?</p>
  
  <p>Thanks in advance</p>