title: Prevent tab change event in angular material md-tab
body: <p>Is it possible to prevent tab change in angular-material's md-tabs or md-tab directive?
  I use the md-on-select directive to execute a function once a tab is clicked and set the md-active value to false for a specific tab. But the tab switch still happens and I seem not to be able to prevent it:</p>
  
  <pre><code>&lt;md-content&gt;
          &lt;md-tabs md-dynamic-height md-border-bottom&gt;
              &lt;md-tab label="Tab1" md-active="ctrl.selectTab1" md-on-select="ctrl.tabClicked('Tab1')"&gt;
                  &lt;md-content&gt;
                      myContent
                  &lt;/md-content&gt;
              &lt;/md-tab&gt;
              &lt;md-tab label="Tab2" md-active="ctrl.selectTab2" md-on-select="ctrl.tabClicked('Tab2')"&gt;
                  &lt;md-content&gt;
                      mycontent
                  &lt;/md-content&gt;
              &lt;/md-tab&gt;
          &lt;/md-tabs&gt;
  &lt;/md-content&gt;
  </code></pre>
  
  <p>In the controller function I have</p>
  
  <pre><code>function tabClicked(tab) {
          switch (tab) {
              case 'Tab1':
                  vm.selectTab1 = true;
                  vm.selectTab2 = false;
                  break;
              case 'Tab2':
                  vm.selectTab1 = false;
                  vm.selectTab2 = true;
                  break;
          }
  }
  </code></pre>