Allow and/or document preprocessor usage with the ACE interface

[goodmami]

gTest allows preprocessors in the ART command like this:

ace_cmd = '{prep}ace -g {gram} {opts}'.format(
            prep=ace_preprocessor + ' | ' if ace_preprocessor else '',
            gram=grm,
            opts=' '.join(ace_options or [])
        )
        subprocess.check_call(
            ['art', '-a', ace_cmd, dest_dir] + (options or []),
            stdout=log, stderr=log, close_fds=True
        )

But it's not possible, or at least not clear how, to use a preprocessor in pyDelphin with the ACE interface directly.