Add type_caster for std::nullopt

[zhqrbitee]

Currently if any code uses std::nullopt, it will throw a bast_cast exception as no type_caster has been defined for it: #274.
As std::nullopt is a standard way to assign empty value to std::optional, so I think it is better to add the type_caster for it in stl/optional.h

[wjakob]

Thanks!