React-native-snap-carousel: [Feature request] flag for whether to trigger callback for snapToItem

[jlo1]

Would like to add flag so that onSnapToItem can be disabled when the carousel is manually snapped via the method snapToItem.
e.g. carousel.snapToItem(4, {:animated true :callback false}) would _not_ trigger the onSnapToItem callback.

Reason being that we want to trigger the callback when the item is snapped via the user swiping, but we don't want to trigger the callback when we programmatically snap to the item.

[bd-arc]

@jlo1 Wish granted in version 3.7.0 ;-)

snapToItem() now accepts an optional third parameter for this use case.