title: Event handler when site doesn't match manifest.json 'matches'
body: <p>My manifest.json contains this block:</p>
  
  <pre><code>"content_scripts": [
      {
          "all_frames": true,
          "matches": [
              "*://www.google.com/*",
              "*://www.cnn.com/*",
              "*://*.foxnews.com/*"
          ],
          "js": ["js/main.js"]
          }
      ],
  </code></pre>
  
  <p>I need an event to fire (either in my content js or background.js, doesn't matter) whenever the user visits a website that does <strong>not</strong> match one of the sites in my <code>matches</code> setting. </p>
  
  <p>How is it done?</p>