Run hooke.command.Argument.callback every time (if defined).

Run hooke.command.Argument.callback every time (if defined).

The previous implemtation which only ran the callback if a value was
given was too confusing.

Also:
  * Added hooke.plugin.playlist.NoteFilterCommand.  In general, I
    think running commands this way is pretty awkward.  We should
    probably just use a Python interpreter instead of cmd.Cmd, but
    for now, NoteFilterCommand provides something like the old
    'copylog' functionality.
  * Removed a bunch of superseded code from hooke.hooke_cli.