title: Chrome Extension get selected text
body: <p>I am looking for a way to get the selected text into my Chrome extension. </p>
  
  <p>I want to ex. select a text in facebook feed and when I click my icon it will get it and show the selected text in my Extension. </p>
  
  <p>I got this so far:</p>
  
  <pre><code>chrome.tabs.executeScript(null, {
    code: "alert(window.getSelection().toString());"
  })
  </code></pre>
  
  <p>it gets the selected text and alert it with a message in Chrome. However I want to show it in my html popup. I want to write it out like this:</p>
  
  <pre><code>document.getElementById("output").value = "Selected text here(but how)"
  </code></pre>
  
  <p>Need help! and I know there is other question about this, but they didn't give me exactly what I want.. </p>