Option for forcing port to listen on

[RBogie]

Currently, librespot listens on 5353 and a random port. This makes running it on a pc with firewall difficult. It would be nice if there was an option to select a specific port instead.

I've never programmed in rust before, so I haven't made a PR for it. Currently locally I solved it by simply putting my port instead of 0 in the following piece of code: TcpListener::bind(&"0.0.0.0:0".parse().unwrap(), handle)?;.