Merge pull request #545 from ccutrer/dynamic_output_device

Read `stdout` from the on-start command to choose ALSA output device

Thanks for this. It looks well-implemented, but it also is very special-purpose. When a more general solution comes along that provides this functionality, I hope you won't mind if this code is removed...

diff --cc audio_alsa.c
Simple merge

diff --cc shairport.c
Simple merge