suggestion: onStart handler

[whitegrey]

onStart handler that runs only once at the beginning of video playing (and no other time, unless video ID changes).

It's a pretty simple handler so I'm sure there's many uses, but my specific use cases are:

• an UI effect that fades in the player as a video begins playing.
• fade in the volume gradually as the video begins playing.

The handler can technically be emulated with onPlay and setState, but it's rather verbose for a relatively fundamental act.

[cookpete]

Added in 0.6.0