Implement datachannel broadcasting

[Azarattum]

Currently data channels created on any end are simply ignored and not forwarded to other peers. This PR establishes data channels that are properly broadcasted from streamer to all the viewers and back.

[Sean-Der]

This is fantastic! Thank you so much for the PR.

I am going to add a simple test and merge it now :)

[Sean-Der]

@Azarattum How important do you think supporting multiple DataChannels is?

This could be a lot simpler if each side could open one DataChannel.

Publishers <-> Subscribers could push messages. If a publisher (or subscriber) hasn't opened a DataChannel the message doesn't get delivered.

What do you think? I love adding DataChannel support. I just want to see if I can lower the amount of code, but enable you to build the thing you care about :)

[Azarattum]

@Sean-Der, I think broadcast-box should be as transparent as possible. So that it could be a drop-in replacement for an implementation that works with direct streaming. And since WebRTC API in browsers supports multiple data channels, I think it would be more appropriate to support them here too. It means that apps written for direct WebRTC streaming could work seamlessly with broadcast-box.

[Sean-Der]

@Azarattum Unfortunately Broadcast Box isn't really a drop in replacement for direct streaming today. It only supports 1 audio and 1 video track. It also supports Simulcast (which you can't do with direct streaming)

I am really excited about DataChannel support and want to add it. However I think it would be better to only support one. It simplifies the code, and we can still build all the same exact stuff with it.

[Sean-Der]

Sorry I accidentally closed the PR :( mind re-opening? I was fixing up stuff and pushing to your repo.

[Azarattum]

@Sean-Der, no problem. Reopened at #385.