title: handling Multiple buttons in $ionicPopup prompt
body: <p>The documentation says:</p>
  
  <blockquote>
    <p>Show a simple prompt popup, which has an input, OK button, and Cancel
    button. Resolves the promise with the value of the input if the user
    presses OK, and with undefined if the user presses Cancel.</p>
  </blockquote>
  
  <p>I was doing:</p>
  
  <pre><code>$ionicPopup.prompt({ 
    //options 
  }).then(function (userinput) {
    //get userinput and send to server
  });
  </code></pre>
  
  <p>I need to add a third button, but can't get the text of the input, how can I resolve the promise on the onTap event of the button to get the input? </p>
  
  <pre><code>  $ionicPopup.prompt({
     title: '¿Are you sure?',
     inputType: 'text',
     buttons: [
     { text: 'Cancel' 
       //close popup and do nothing
     },
     {
      text: 'NO',
      type: 'button-assertive',
      onTap: function(e) {  
        //send to server response NO
      }
     },
     {
      text: 'YES',
      type: 'button-energized',
      onTap: function(e) { 
        //get user input and send to server
      }
     }]
  </code></pre>