title: Highlight code in knitr .RMD document
body: <p>Ive been reading through the Rstudio Markdown tutorials trying to figure out how to do syntax highlighting on my code in a .rmd file, but cant figure it out. Anyone know how to do it? My output goal right now is .html, but that can change if needed. Thanks!</p>
  
  <p>A small code snippet of what I tried. The page text works fine but the code does not highlight:</p>
  
  <pre><code>---
  title: Eve Historical Market Analysis
  author: 
  date: 12/3/18
  output:
    prettydoc::html_pretty:
      theme: cayman
      highlight: github
  ---
  ```
  library(rmarkdown) #used for syntax highlighting in this document
  library(shiny)
  ```
  #read in file of item ids from website and parse
  ```
  url = "http://eve-files.com/chribba/typeid.txt"
  df = read_fwf(url, fwf_empty(url), skip = 2)
  colnames = read_table(url, n_max = 1)
  names(df) = names(colnames)
  </code></pre>