Add uPnP/DLNA casting support

[dweymouth]

Ability to cast playback to remote speakers and devices implementing the uPnP/DLNA protocol

[sochotnicky]

I wonder about supporting Chromecast as well. Probably worth a separate issue though I suspect there might be some deduplication? I think both basically require that we start a http server to actually serve the content.

Chromecast might actually be simpler?

[dweymouth]

Created #246 to track adding Chromecast support. I had made the uPnP/DLNA ticket first because I'm aware of this project which is also written in Go and uses DLNA. Also, I personally don't have any Chromecast devices to test with, but if someone wanted to give Chromecast a go I'd happily review a PR