Callback for prev/next button click

[pvn-anh-pham]

This is really cool project, I already integrated this one into my app. It has almost functional but I hope you could add callback for prev/next button click event. That help the app can control more from your lib.

Ex: app can preload the next section when user at the end of a chapter and press next button

That is worth to fight, right !?

Thank for your hard working!