Ability to use a custom components instead of Photo for items on the stream

[SammyIsra]

If you pass a component to <Photostream /> so that if exists, use that instead of <Photo />

This can be an alternate to #8. If we pass a custom component, that component can have its own event handlers.

[SammyIsra]

Remember to include documentation on how to deal with event handlers, and what props are passed to the custom component.