tmux support and domain socket support

[mixu]

Would be nice to have a way to integrate nplay more closely with tmux, so that I could have a status bar on tmux with the current song as well as the option to use tmux to control nplay.

This could be done via a unix domain socket that speaks http, allowing the same socket interface to also be used on Windows to do remote playback over REST / over whatever tool I'll write to control it, with minimal extra code. Would skip the need to install a Windows ssh server just to do remote playback...