Allow playback of local resources

[pennersr]

I need playback of local resources. This pull adds support for this. You can refer to local resources for both the video and the poster like this:

<Video src="resource:video.mp4" poster={require('./foo.png')} .../>

You'll need to add the video file to your project:

• iOS: Simply add it as a resource in Xcode
• Android: Drop it in android/app/src/main/assets/video.mp4