Added new 'all directories - metadata module - rewrite (mdmrw)' command.

Description: use (mdmrw) to rewrite *ALL* metadata modules everywhere. The
command visits each of these four storehouses ...

    * Abjad material packages directory
    * Abjad score packages directory
    * User library
    * User score packages directory

... descends each storehouse and then rewrites every __metadata__.py
module found.