Do not block reading stdin if the user passes --help

[Marius Gedminas]

I was wondering if black-macchiato would allow me to specify -S to pass through to black (so it would skip string literal normalization), so naturally I ran black-machiatto --help to see if it accepts options.

Instead black-machiatto blocked reading stdin from the terminal.

It would be great if black-machiatto handled this a little bit better. E.g. it could check whether sys.argv contains -h/--help and then print a short help message telling the user that all command-line arguments will be passed to black.