add mpd_current_song

- mpd_current_song is bar item for showing currently playing song on
  MPD. there's also a bar item for that in mpdbitl script, but that
  script requires bitlbee. this one just needs mpc.

- fixed script to "correctly" use weechat's "constant". see
  http://git.savannah.gnu.org/gitweb/?p=weechat.git;a=commitdiff;h=2ffa199a7d3f694694e5051726f497f3492d1bfc