Add Support for Encrypted Channels

[sandstrom]

Looking forward to Ruby support for encrypted channels:
https://pusher.com/docs/channels/using_channels/encrypted-channels#library-support

Minor idea:
What if you'd allow adding the encryption key to your web debug UI, client-side (JS-side), that way we could read/inspect messages in encrypted channels without you knowing about the key. It would be neat and (perhaps) not too difficult to implement. Obviously the key would be lost if the page was reloaded. But still fairly useful.

[sandstrom]

@callum-oakley Awesome! 🎉