[Clopts] Add option to ignore or warn on unknown arguments

Something like:
```c++
unknown_error // default, obviously
unknown_warn
unknown_ignore
```

Implement this as a type that stores an enumerator; we could expose the enumerator, but that would mean 1. more things to check, and 2. `unknown<unknown_mode::error>` is too verbose, and `unknown<error>` is just namespace pollution, so there’s no particularly good way of doing that...