title: Need to get selected text as a string from the current tab
body: <p>Here's the code I currently have:</p>
  
  <pre><code>var clicked = function(){
      var selection
      chrome.tts.speak(selection, {'lang': 'en-US', 'rate': 0.8, 'enqueue': true});
  }
  chrome.contextMenus.create({
    "title" : "Say '%s'!",
    "type" : "normal",
    "contexts" : ["selection"],
    "onclick" : clicked()
  });
  </code></pre>
  
  <p>I need to get the selection from the current tab. I don't know how to get it.
  Can anyone help?</p>