Add possibility to render an overlay component on video

[mlazari]

This change adds a new renderOverlayComponent prop that allows to render something on top of the video, but under controls / play button.

Example:

        <VideoPlayer
          video={{uri}}
          style={styles.video}
          videoWidth={320}
          videoHeight={240}
          renderOverlayComponent={() => (
            <Text
              style={{
                color: 'white',
                position: 'absolute',
                right: 20,
                bottom: 20,
              }}>
              Testing
            </Text>
          )}
        />