Removes tabindex property from all links on the current slide, sets
tabindex = -1 for all links on other slides. Prevents slides from appearing
out of control.
function updateTabIndex() {
  var allLinks = document.querySelectorAll('.slide a');
  var position = currentPosition();
  var currentPageLinks = document.getElementById('slide-' + position).querySelectorAll('a');
  var i;

  for (i = 0; i < allLinks.length; i++) {
    allLinks[i].setAttribute('tabindex', -1);
  }

  for (i = 0; i < currentPageLinks.length; i++) {
    currentPageLinks[i].removeAttribute('tabindex');
  }
}