Display / set / update the user email.

def email(email_address):
    """Display / set / update the user email."""
    if not email_address:
        click.secho(dtool_config.utils.get_user_email(CONFIG_PATH))
    else:
        click.secho(dtool_config.utils.set_user_email(
            CONFIG_PATH,
            email_address
        ))