title: Rmarkdown PDF citations not rendering correctly (csl issue?)
body: <p>I'm getting to grips with writing publications in rmarkdown. No issues at all generating .html files, or .docx files, but when I generate .pdf files the citations don't seem to inherit the style defined in the .csl file.</p>
  
  <p>For example, with a numbered .csl style I expect:</p>
  
  <p><code>[@Author_Title_2003]</code> -> <em>(1)</em></p>
  
  <p>Which is successful in .html and .docx files, but in .pdfs I get:</p>
  
  <p><code>[@Author_Title_2003]</code> -> <em>[Author, 2003]</em></p>
  
  <p>With square brackets printed as well.</p>
  
  <p>An example:</p>
  
  <p>test.rmd:</p>
  
  <pre><code>---
  title: 'My Title'
  author: "Me me me me!"
  output: pdf_document
  bibliography: references.bib
  csl: elsevier-vancouver.csl
  ---
  
  Application written in the R programming language [@RCoreTeam] using the Shiny framework [@Chang2015].
  
  # REFERENCES
  </code></pre>
  
  <p>references.bib:</p>
  
  <pre><code>@Misc{Chang2015,
    Title                    = {shiny: Web Application Framework for R. R package version 0.12.1},
    Author                   = {Chang, W. and Cheng, J. and Allaire, JJ. and Xie, Y. and McPherson, J. },
    Year                     = {2015},
    Type                     = {Computer Program},
    Url                      = {http://CRAN.R-project.org/package=shiny}
  }
  @Article{RCoreTeam,
    Title                    = {R: A Language and Environment for Statistical Computing},
    Author                   = {{R Core Team}},
    Year                     = {2015},
    Type                     = {Journal Article},
    Url                      = {http://www.R-project.org}
  }
  </code></pre>
  
  <p>elsevier-vancouver.csl: <a href="https://github.com/citation-style-language/styles/blob/master/elsevier-vancouver.csl" rel="nofollow noreferrer">link</a></p>
  
  <p>running <code>rmarkdown::render("test.Rmd", "pdf_document")</code> gives:</p>
  
  <pre><code>/home/jordan/.cabal/bin/pandoc +RTS -K512m -RTS paper.utf8.md --to latex --from markdown+autolink_bare_uris+ascii_identifiers+tex_math_single_backslash-implicit_figures --output paper.tex --template /home/jordan/R/x86_64-pc-linux-gnu-library/3.2/rmarkdown/rmd/latex/default-1.14.tex --highlight-style tango --latex-engine pdflatex --natbib --variable graphics=yes --variable 'geometry:margin=1in' --bibliography references.bib 
  </code></pre>
  
  <p>And the output file is:</p>
  
  <p><a href="https://i.stack.imgur.com/gQOOD.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/gQOOD.png" alt="Bad PDF output"></a></p>
  
  <p>See the incorrectly formatted citations. Also note this format is generated regardless of the csl argument in the header. Any help would be much appreciated.</p>
  
  <p>pandoc version 1.15.2.1, pandoc-citeproc version 0.8.1.3.</p>