Add support for a post-processor hook.

Add support for a post-processor hook.

(fa-flacd):
    Document the hook.

    (flac): Execute and remove the hook if it exists before removing
    the temporary directory.

(fa-rip):
    (make_post_processor): New function to create the post-processor
    file, if defined.

    (MAIN): Take new -p/--post-processor option.