Add `# docstub: ignore` comment directive

[lagru]

Similar to # docstub: on ... #docstub: off it would be nice to have a directive to disable transformation for one line. E.g., this would be useful to preserve a decorator such as click.command that's typing relevant but from a third-party package.

Though, for now something like

# docstub: off
@click.command()
# docstub: on
@click.option(...)
def main(*args, **kwargs):
    ...

works too. So not of utmost priority.