Add handling for libnotify notifications in mpd

Added handling for libnotify notifications in the mpd status section.
This adds two other options to the mpd config section -
notif_header_format and notif_body_format, which share format options
with the format options for the mpd status line.
Unfortunately, I think I have to duplicate this

Unfortunately, it looks like I have to duplicate this code in order to
make things work properly. If someone can help out with this that'd be
awesome