publish: Polish/refactor the publishing workflows. * Add a `--preview` option. * The `--list` option gives a nicer output, and prints warnings/errors for incorrect configuration. * Moved most of the `shell` code into a base class that's reusable. * Simplified the code to log publishing to a file. * Nicer output overall, with times.

from piecrust.serving.server import WsgiServer


def get_app(root_dir, sub_cache_dir='prod', enable_debug_info=False):
    app = WsgiServer(root_dir,
                     sub_cache_dir=sub_cache_dir,
                     enable_debug_info=enable_debug_info)
    return app