title: use csl-file for pdf-output in bookdown
body: <p>I would like to use a <code>.csl</code>-file for formatting references with bookdown. Adding <code>csl: some-style.csl</code> to <code>index.Rmd</code> affects the output to <code>gitbook</code>, but not to <code>pdf_book</code>. I know that I can specify <code>biblio-style</code>, but this only accepts some standard styles and not csl-files. Is there a proper workaround?</p>
  
  <p>Steps to reproduce:</p>
  
  <ol>
  <li>Create new project with RStudio and choose "Book Project using bookdown" as option.</li>
  <li>Download some <code>.csl</code> file from <a href="https://www.zotero.org/styles" rel="noreferrer">https://www.zotero.org/styles</a> and copy to root of project.</li>
  <li>Add <code>csl: my_csl_file.csl</code> to the header in <code>index.Rmd</code>. </li>
  <li>Build the book to pdf and html, and observe the differences in the references (either in the references section, or in the introduction)</li>
  </ol>
  
  <p>Header in <code>index.Rmd</code>:</p>
  
  <pre><code>--- 
  title: "A Minimal Book Example"
  author: "Yihui Xie"
  date: "`r Sys.Date()`"
  site: bookdown::bookdown_site
  documentclass: book
  bibliography: [book.bib, packages.bib]
  csl: american-sociological-review.csl
  link-citations: yes
  description: "This is a minimal example of using the bookdown package to write a book. The output format for this example is bookdown::gitbook."
  ---
  </code></pre>
  
  <p><strong>HTML output (correct):</strong>
  <a href="https://i.stack.imgur.com/9yvr6.png" rel="noreferrer"><img src="https://i.stack.imgur.com/9yvr6.png" alt="enter image description here"></a></p>
  
  <hr>
  
  <p><strong>PDF output (incorrect):</strong>
  <a href="https://i.stack.imgur.com/kEZqF.png" rel="noreferrer"><img src="https://i.stack.imgur.com/kEZqF.png" alt="enter image description here"></a></p>
  
  <p><a href="https://i.stack.imgur.com/MM2lR.png" rel="noreferrer"><img src="https://i.stack.imgur.com/MM2lR.png" alt="enter image description here"></a></p>