Add ability to specify preloadAdditionalHeight and preloadBatchSize through scale factors relative to the containerHeight

When working with the window as the scroll container, it is sometimes useful to specify a scale factor relative to the container height as the batch size, so your code does not need to know anything about the window. To do this, use Infinite.containerHeightScaleFactor. So, for example, if you want the preloaded batch size to be twice the container height, write preloadBatchSize={Infinite.containerHeightScaleFactor(2)}. The same goes for preloadAdditionalHeight - additional information can be found in the documentation.