Set a default stream for clients to choose

[genericuser256]

Is your feature request related to a problem? Please describe.
By default new clients choose the first non-meta stream to connect to. For my use case, it's helpful to be able to set the default stream for a client to connect to, specifically I'd like my one meta stream to be the default that clients connect to.

Describe the solution you'd like
I think the easiest solution here is to add a setting to snapserver.conf:stream of default_source that takes the name of the stream to use as the default.

Describe alternatives you've considered
Another option is to allow setting priority per stream which would be easiest if people were commonly adding and removing streams, but as that feels unlikely I figure this is the best approach.

[genericuser256]

I have a working change here if desired: https://github.com/genericuser256/snapcast/tree/add-default-source