Support custom children as layer content for ParallaxBanner

Should be possible to allow custom children for each layer of the `<ParallaxBanner>`. This would allow video or other html to parallax inside a banner. See #21

```jsx
const Video = ({ src }) => (
    <video src={src}></video>
);

...

<ParallaxBanner
    layer={[{
        amount: 0.2,
        children: <Video src="foo.mp4">,
    }]}
/>
```
