To demonstrate the new I/O thread. libsoup is well-integrated into
the GLib main loop, which made this plugin pretty easy to write.
As a side effect, we have to initialize the I/O thread in all debug
programs that use the input API.
For Spotify tracks. Uses a spt URI, so with mpc you can play tracks
with e.g.,
mpc add spt://spotify:track:5qENVY0YEdZ7fiuOax70x1
mpc play
Uses the pcm_decoder_plugin for the output
