optional handling for unhandled options

[jwinarske]

I have a case where I want to store unhandled options in a std::vector<std::string> and use later.

I'm thinking adding a std::vector<std::string> parameter to the parse method with a default value of nullptr would suffice. If parameter was not nullptr the unhandled options would be stored in the vector reference. If nullptr, it would follow the existing logic.

Would you accept a PR for this?