title: disable prev button in bootstrap tour
body: <p>Have a tour like this</p>
  
  <pre><code>var TourToolTipPoolConfiguration = new Tour({
              name: 'TourToolTipPoolConfiguration',
              steps: [
                  {
                      onNext: function (TourToolTipPoolConfiguration) {
                          TourToolTipPoolConfiguration.goTo(3);
                      },
                      element: "#TimeExpireTicket",
                      title: "foo",
                      content: "test1"
                  },
                  {
                      element: "#DayNumber",
                      title: "foo",
                      content: "test2"
                  },
                  {
                      element: "#SavePoolConfig",
                      title: "foo",
                      content: "test3"
                  }
              ]
          });
  </code></pre>
  
  <p>step1 or step2 is optional, i hidde the div display:none, but if tour init in step 2, i need disable prev button because step1 is hidden, how make that? i try onPrev evaluate if div if visible but not work</p>
  
  <p><a href="https://i.stack.imgur.com/lXrYk.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/lXrYk.png" alt="enter image description here"></a></p>