Stop parsing options/flags at literal `--`

[Lawrence Chan]

Many argument parser implementations (including python argparse) treat a literal -- as a marker that breaks out of full argument parsing and collects the remaining args as positional/remaining arguments. Correct me if I'm wrong, but this library does not support that out of the box. I was able to sort of emulate it with an explicit -- remaining-argument in a mutually exclusive group with a positional-remaining argument, but that adds a bunch of extra complexity to the implementation as well as extra noise in the help/usage message.