Observing onScrollToPage event

[me-abhinav]

It will be useful if there is a function prop for observing the scroll events like onScrollToPage / onScrollToItem. e.g.

<Carousel
    ref={'carousel'}
    onScrollToPage={(page) => {
        console.log(page);
    }}
    items={this.state.entries}
    renderItem={this._renderItem}
    sliderWidth={sliderWidth}
    itemWidth={itemWidth}
    slideStyle={styles.slide} />