Keep running even when MPD isn't

In my current setup ‘mpd-notify’ gets started before ‘mpd’ is fully started, so
at boot it'll immediately get disabled and I'll frequently forget to turn it
back on.

mpd-notify

@@ -1,5 +1,5 @@
 #!/usr/bin/env zsh
-mpc idleloop player | while read _
+while ! mpc idleloop player; do sleep 1; done 2>/dev/null | while read _
 do
     song=$(mpc current)
     state=$(mpc status %state%)