on_new_slide Callback Option Implemented

[f3ndot]

I personally found it useful for the plugin to provide a callback option when a new slide / image is displayed. I've written in support under the option name of "on_new_slide". Demo of this is in demo.html and I've updated the version number to 3.2.8 as a result of this addition.

First time doing this, so let me know if I've done something wrong. I believe it's alright.

<script type="text/javascript">
    function callback() {
        alert('success!');
    }
    jQuery(function($){
        $.supersized({
            on_new_slide    :   callback,   // Calls a function when a new slide/image is loaded
            slides          :   [
                {...}
            ]
        });
    });
</script>