[Feature] 'currentSticky' && OnStickHeaderChanged()

[android-feng]

Hi there:

Thanks for this great lib. It really helped us a lot during ur work. And these two more features below are some functions I guess might be useful to other developers who also like to use it during daily work. So, sorry for bothering and please review 😄

Reason:

1. [currentSticky] the position value OnHeaderClickListener returned is not correct when the header view is sticky, it always returns 1. Thus, I guess a way out is adding this 'currentSticky' to Listener's callback. And then when sticky, we can just use recyclerview LayoutManager's findFirstVisibleItemPosition()first item to do this hack.
2. OnStickHeaderChanged() could be used to change Stick Header's details when header changed

@timehop-deploy @jacobtabak @emanuelet Plz Review 😄