title: Merging two collections using Underscore.JS
body: <p>Provided I have two collections:</p>
  
  <p>c1 - <code>[{a:1},{a:2},{a:3}]</code></p>
  
  <p>and</p>
  
  <p>c2 - <code>[{a:1},{a:7},{a:8}]</code></p>
  
  <p>what's the fastest way to add unique items from <code>c2</code> into <code>c1</code> using <code>Underscore.JS</code>? The real numbers in collections would be <code>2K</code> for <code>c1</code> and <code>500</code> for <code>c2</code>, the operation is performed often, so must be performant!</p>
  
  <p><strong>Update 1</strong> - I'm only using <code>Underscore.JS</code> for a few days, I couldn't find a way to add one collection into another (I can filter <code>c2</code> myself) - is that trivial in <code>Underscore.JS</code>?</p>