title: Using Rmarkdown with CSS from CDN
body: <p>I want to create a Rmarkdown using the Bootstrap CSS from CDN (<a href="http://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/css/bootstrap.min.css" rel="nofollow">http://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/css/bootstrap.min.css</a>).</p>
  
  <p>I'm trying to do this, but it didn't works...</p>
  
  <p>This is what I tried (I tried without using NULL, and without " " too, but I had the same error)</p>
  
  <pre><code>---
  title: "Habits"
  output:
    html_document:
      theme: NULL
      highlight: NULL
      css: "http://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/css/bootstrap.min.css"
  
  ---
  </code></pre>
  
  <p>The resulting error is:</p>
  
  <pre><code>"E:/Program Files/R/RStudio/bin/pandoc/pandoc" +RTS -K512m -RTS teste.utf8.md --to html --from markdown+autolink_bare_uris+ascii_identifiers+tex_math_single_backslash-implicit_figures --output teste.html --smart --email-obfuscation none --self-contained --standalone --section-divs --template "C:\Users\diego\Documents\R\win-library\3.2\rmarkdown\rmd\h\default.html" --css "http:\\maxcdn.bootstrapcdn.com\bootstrap\3.3.6\css\bootstrap.min.css" --mathjax --variable "mathjax-url:https://cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS-MML_HTMLorMML" --no-highlight 
  output file: teste.knit.md
  
  pandoc.exe: Could not fetch http:\\maxcdn.bootstrapcdn.com\bootstrap\3.3.6\css\bootstrap.min.css
  InvalidUrlException "http:%5C%5Cmaxcdn.bootstrapcdn.com%5Cbootstrap%5C3.3.6%5Ccss%5Cbootstrap.min.css" "URL must be absolute"
  Error: pandoc document conversion failed with error 67
  Besides that: Warning message:
  command execution '"E:/Program Files/R/RStudio/bin/pandoc/pandoc" +RTS -K512m -RTS teste.utf8.md --to html --from markdown+autolink_bare_uris+ascii_identifiers+tex_math_single_backslash-implicit_figures --output teste.html --smart --email-obfuscation none --self-contained --standalone --section-divs --template "C:\Users\diego\Documents\R\win-library\3.2\rmarkdown\rmd\h\default.html" --css "http:\\maxcdn.bootstrapcdn.com\bootstrap\3.3.6\css\bootstrap.min.css" --mathjax --variable "mathjax-url:https://cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS-MML_HTMLorMML" --no-highlight' teve status 67 
  Execution interrupted
  </code></pre>
  
  <p>There is some way to refer to a online CSS with a absolute path?</p>
  
  <p>Thanks,</p>