1.3.1

Changes:

• Add method .try_emplace.
• Fix compilation error in opt::option<T>::(const_)iterator when T is a reference type (#2).
• Fix non-working preprocessor macro OPTION_USE_BUILTIN_TRAITS.
• Optimize for compile time algorithm of searching maximum max_level value used in built-in option traits (e.g. for std::tuple, reflectable types).
• Add static_assert check for reference options when constructing them from .map and passed function returns a prvalue.
• Add OPTION_VERIFY check for null pointer dereferencing in .get_unchecked in reference options.