How to make navigation alphabet increment

[mlgiarra]

$('.example-3').flexipage({
pager : false,
navigation : true
});

Example-3 is great, but I need alphabet instead of numbers. Can code be added to increment by letter?

//if navigation is set to true
if (opts.navigation == true) {
var navigationHTML = "";
var actual;

    for (var i = 1; i <= opts.total_pages; i++){
      (opts.firstpage == i)? actual = ' class="active" ' : actual = '';
      navigationHTML += '<li'+actual+'><a rel="'+i+'" href="#">'+i+'</a></li>';
    };

    $letter = "a";
    for (var navigationHTML = $letter; $letter++;){  ...whatever   

    buildPager($target, navigationHTML)