Google Cast doesn't work with multiple devices simultaneously

[Alex Bainter]

Need to support streaming to multiple listeners

Relevant code:

• https://github.com/generativefm/play/blob/06b10153122d63e582eae7361bbc841c956d2215/src/cast/cast-middleware.js
• https://github.com/generativefm/cast/blob/master/src/index.js