implement callback function feature

[tjnet]

What's this PR do?

add callback function.

When the image is loaded, the function defined as 'onLoadCallback' is called.

lazyload(null, {
    onLoadCallback: function() {
        console.log('callback function is called!');
    }
});

Related Issues

#389

[tjnet]

I'm new to this. Is this correct? Please review in your free time. 😄

[Oxicode]

+1

[q2apro]

Seems to be here: https://github.com/tuupola/lazyload/pull/429/commits (not added to the core yet)

File: https://github.com/tuupola/lazyload/blob/2a4fbfc0b702057338724f8ed2cd09c7db17f403/lazyload.js

Further, see here: https://stackoverflow.com/q/11398005/1066234

And with version 1.x: https://github.com//issues/246 - File: https://github.com/tuupola/lazyload/blob/1.x/jquery.lazyload.js