FMT C++ library: allow user to set format specifiers for custom type

The easiest solution is to inherit formatter<custom_type> from formatter<double>:

template <> struct fmt::formatter<custom_type> : formatter<double> {
  auto format(custom_type c, format_context& ctx) {
    return formatter<double>::format(c.value, ctx);
  }
};

https://godbolt.org/z/6AHCOJ