replace “renderEmptyView” with a configurable placeholder

[EyMaddis]

Allows fixes for #23

Allows for implementations like:

          renderBackgroundPlaceholder={({ animatedValue }) => {
            return (
              <FancyHeader
                width={width}
                style={Styles.fancyHeader}
                contentStyle={Styles.fancyHeaderElements}
                rightElement={headerRight}
                leftElement={headerLeft}
                leftAnimatedOpacity={animatedValue.interpolate({
                  inputRange: [0, headerThreshold - 40],
                  outputRange: [1, 0],
                  extrapolate: 'clamp'
                })}
                maskColor={BaseColors.screenBackground}
              />
            )
          }}

I am not super happy with the description in the Readme, maybe a core contributor can clear things up.

[z4o4z]

Hi @EyMaddis, thanks again for great PR, you are doing really nice things!

[z4o4z]

This feature available in the 1.8.0 version!