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.

import os


def slashfix(path):
    if path is None:
        return None
    if isinstance(path, str):
        return path.replace('/', os.sep)
    fixed = []
    for p in path:
        fixed.append(p.replace('/', os.sep))
    return fixed