title: R markdown link is not formatted blue when knitted to pdf
body: <p>for some reason no link in my R markdowns (rmd) is formatted blue. knitting the simple rmd below to pdf is leaving the text color black. only when hovering over it does one realize that it's actually a link. knitting it to html will make the link blue. of course I can use a latex wrapper but I wonder why that is?</p>
  
  <blockquote>
    <p>sessionInfo()
    R version 3.3.0 (2016-05-03)
    Platform: x86_64-w64-mingw32/x64 (64-bit)
    Running under: Windows 7 x64 (build 7601) Service Pack 1
    loaded via a namespace (and not attached):
     knitr_1.15</p>
  </blockquote>
  
  <p>RStudio 1.0.44</p>
  
  <pre><code>---
  title: "R Notebook"
  output:
    pdf_document: default
    html_notebook: default
  ---
  
  ```{r, echo=F}
  # tex / pandoc options for pdf creation
  x &lt;- Sys.getenv("PATH")
  y &lt;- paste(x, "E:\\miktex\\miktex\\bin", sep=";")
  Sys.setenv(PATH = y)
  ```
  
  [link](www.rstudio.com)
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/awos7.png" rel="noreferrer"><img src="https://i.stack.imgur.com/awos7.png" alt="enter image description here"></a></p>