Add option srcAttribute

[emri99]

Allow to define the element attribute to get image url from.

This can be used to preload images used as CSS background:

<style>
.preload-me {
  background-image: url('path/to/loader.gif');
  background-repeat: no-repeat;
  background-size: contain;
} 
</style>
<div class="preload-me" data-image="/image/to/load.png"></div

$('.preload-me').preload({
  srcAttribute: 'data-image',
  onComplete: function(data) {
    if (data.found) {
       data.original.style.backgroundImage = 'url('+data.image+')';
    }
  }
});