Missing fmt::make_printf_args

[Kronuz]

I had to add Added fmt::make_printf_args to fmt/printf.h so I could pass arguments to fmt::vsprintf, as in

fmt::vsprintf(format, fmt::make_printf_args(std::forward<Args>(args)...));

This is my implementation of it:

template <typename ...Args>
inline arg_store<printf_context<internal::buffer>::type, Args...> make_printf_args(const Args & ... args) {
  return arg_store<printf_context<internal::buffer>::type, Args...>(args...);
}

[vitaut]

Could you submit a PR?