version 4.0.3

Fixed

• remove default template argument from forward declaration in toml_fwd.hpp
• enable to call make_error_info with multiple toml::values
• enable to copy/move result with std::reference_wrapper
• fix document generation error with the latest version of hugo
• fix several tiny errors that causes warning
• fix CMake compatibility warning

Changed

• add -Werror / /WX to build script
• set MSVC warning level to /W4
• add explanation of features to README