Handle commercial type to video playback.

[Omsad]

When a video playback message of type commercial is received, the VideoPlayback message data treats it as an StreamUp message as that's the default and there is no type handling it in the type switch.

So I've added a "Commercial" video playback type, an OnCommercial event in the TwitchPubSub class and then wired it altogether.

To do so I had to modify the VideoPlayback class to have a Length property and then create an OnCommercialArgs class.

[Omsad]

Tested yes, in use not at the moment but it will be in the next release of the software I create.

[swiftyspiffy]

Thanks :)