title: Specifying multiple simultaneous output formats in knitr (new)
body: <p>Can I write a YAML header to produce multiple output formats for an R Markdown file using knitr? I could not reproduce the functionality described in the answer for <a href="https://stackoverflow.com/questions/25078572/specifying-multiple-simultaneous-output-formats-in-knitr">the original question with this title</a>.</p>
  
  <p>This markdown file:</p>
  
  <pre><code>---
  title: "Multiple output formats"
  output: 
      pdf_document: default
      html_document:
        keep_md: yes
  ---
  
  # This document should be rendered as an html file and as a pdf file
  </code></pre>
  
  <p>produces a pdf file but no HTML file.</p>
  
  <p>And this file:</p>
  
  <pre><code>---
  title: "Multiple output formats"
  output: 
    html_document:
      keep_md: yes
    pdf_document: default
  ---
  
  # This document should be rendered as an html file and as a pdf file
  </code></pre>
  
  <p>produces an HTML file (and an md file) but no pdf file.  </p>
  
  <p>This latter example was the solution given to the <a href="https://stackoverflow.com/questions/25078572/specifying-multiple-simultaneous-output-formats-in-knitr">original question</a>. I have tried knitting with Shift-Ctrl-K and with the Knit button in RStudio, as well as calling <code>rmarkdown::render</code>, but only a single output format is created, regardless of the method I use to generate the output file.</p>
  
  <p>Possibly related, but I could not identify solutions:</p>
  
  <ul>
  <li><a href="https://stackoverflow.com/questions/31214524/how-do-i-produce-r-package-vignettes-in-multiple-formats">How do I produce R package vignettes in multiple formats?</a></li>
  <li><a href="https://github.com/yihui/knitr/issues/1051" rel="noreferrer">Render all vignette formats #1051</a></li>
  <li><a href="https://github.com/yihui/knitr/issues/769" rel="noreferrer">knitr::pandoc can't create pdf and tex files with a single config #769</a></li>
  <li><a href="https://github.com/yihui/knitr/issues/547" rel="noreferrer">Multiple formats for pandoc #547</a></li>
  <li>An allusion to multiple output format support in a <a href="https://blog.rstudio.org/2014/06/18/r-markdown-v2/#comment-10813" rel="noreferrer">three year old RStudio blog post</a></li>
  </ul>
  
  <p>Using R version 3.3.1 (2016-06-21), knitr 1.14, Rmarkdown 1.3</p>