This adds two new parsing options for argument arrays, and renames
`.unconditionalRemaining` to `.captureForPassthrough`.

- `.allUnrecognized` collects all the inputs that weren't used during
parsing. This essentially suppresses all "unrecognized flag/option"
and "unexpected argument" errors, and makes those extra inputs
available to the client.
- `.postTerminator` collects all inputs that follow the `--`
terminator, before trying to parse any other positional arguments.
This is a non-standard, but sometimes useful parsing strategy.