mpc song number completion

[Matt Smith <mreedsmith@xxxxxxxxx>]

mpc version 0.18 removed song numbers from the output of `mpc playlist`,
breaking zsh completion of song numbers such as in `mpc play`. The old
behavior can be restored by adding "%position%) " to the beginning of the
--format string (this variable was introduced in 0.19).
One-liner:
zstyle ':completion:*:*:mpc:*' mpc-format-string "%position%) %file%"