use `show_default` for Click.options

instead of manually writing things like 'default is None' in help strings for options, use the show_default=True kwarg for Click.options which will automatically display the correct default alongside the help message. this should not be set for options where the default would be nonsensical though; for example, if the default for a callable is None and the side effect of that is to fallback to a different callable then the help message should document that instead. This must not break current formatting layout limits