Add option to ignore or warn on unknown arguments

[Sirraide]

Something like:

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...