Feature request - add option: callback function

[Volderr]

Actually I have added this functionality directly to the plugin code - but it would be much better if it would be as an input parameter.

callback function - user could define function that should trigger after the elements stop moving inside container.

This is useful when you for example need to upload something dynamically, and after each scroll occur you need to check whether you have reached the end or not yet.

[azoff]

Implemented in 1.3.4