Added onSlide function

[kickbk]

Adding the ability to listen to the FlatList onSwipePageChange event by adding the onSlide option to the component. It retrieves the current and previous page indexes. Example:

<Onboarding
    onSlide={({currentPage, previousPage}) => {
      ...
    }}
   ...

[jfilter]

Hey, thanks for proposing, but there is already an pageIndexCallback that returns the new page index on change. The current and the previous page can be derived from the page index change.