adds callback option

[putchi]

If "callback" (function) option isset, it will be called when done printing or when canceling printing.

• tested and working great.
• if yow will approve this pull request please update the example :)

[CitizenOfRome]

Thank you very much for taking the time to create this pull request.

The plugin already has the deferred option where you can pass a jQuery Deferred object with multiple callback functions attached.

And because we lack a reliable way to know if the printing failed, the Deferred object is always resolved after the actual printing has been triggered.

Hope this makes sense?