String encoding should allow UTF-8 output

[tgockel]

All std::string -> JSON string conversion goes through the jsonv::detail::string_encode function, which performs numeric encoding for characters which do not fit into the ASCII encoding. This is not entirely necessary, since a JSON document can validly contain UTF-8 sequences of characters. The library should allow replacement of this encoding function if the user knows the decoding side can handle UTF-8 encoded JSON.