title: Rendering Appendix Figure Numbers in Bookdown
body: <p>Bookdown is a great package and I look forward to seeing how it developes, but right now I am having troubling rendering figure numbers in the <code>pdf_document2</code> format when figures are in appendices. Specifically when a figure with a caption is in an appendix the figure number should be of the form A.1, A.2, B.1, B.2 etc. but instead the figure numbers are treated as normal sections with numbers 3.1, 3.2, 4.1, 4.2 etc. where the appendices are the third and fourth sections respectively. Here is an example:</p>
  
  <pre><code>---
  title: "Untitled"
  author: "John Doe"
  date: "November 18, 2016"
  documentclass: article
  output:
    bookdown::pdf_document2:
      toc: yes
      fig_caption: yes
      number_sections: yes
  linkcolor: blue
  ---
  
  # Chapter One
  
  ```{r a, fig.cap="rabble rabble"}
  plot(cars) # a scatterplot
  ```
  
  ```{r b, fig.cap="rabble rabble"}
  plot(cars) # a scatterplot
  ```
  
  # Chapter Two
  
  # (APPENDIX) Appendix {-}
  # Appendix A
  
  ```{r c, fig.cap="rabble rabble"}
  plot(cars) # a scatterplot
  ```
  
  # Appendix B
  
  ```{r d, fig.cap="rabble rabble"}
  plot(cars) # a scatterplot
  ```
  </code></pre>
  
  <p>Is this an issue with bookdown itself, or is it incorrect in some way?</p>