Allow for custom placeholder view to be provided via props.

[ognen]

The usage should be:

<TweetView tweetId={323} placeholder={<Placeholder />} />

alternatively

<TwieetView tweetId={332} renderPlaceholder={() => <Placeholder />} />

[gorjanz]

Right now, the Placeholder view is managed from the native module - VAMP-924 / VAMP-923
@ognen Can you please support @cristisamoila and @andianton in implementing this issue, because I don't have enough experience in this area. Also, you can decide whether the native placeholder view is enough for start.