title: Setting document title in Rmarkdown from parameters
body: <p>I've got an Rmarkdown template that works well, and I've parameterized it so I can generate variants of the same report from different data sources. However, I'd like to change the title of the report in each case. How do I do that? </p>
  
  <p>Here's the YAML header I have so far:</p>
  
  <pre><code>---
  title: "My Title"
  author: "Me, Inc."
  date: "August 4, 2015"
  output: pdf_document
  params:
    title: default
  ---
  </code></pre>
  
  <p>I've tried using <code>params=list(title="ASDF")</code> in the call to <code>rmarkdown::render</code>, and although my code can see that variable, it doesn't change the title. I've also tried using <code>r params$title</code> in the YAML, but that gives a syntax error.</p>
  
  <p>Is there something else I should be trying? Thanks!</p>