Add crow::json::returnable class

[CarloDG80]

• Add crow::json::returnable class
  An utilty to return a JSON string build by an external library/tool.

• Add "operator double()" to crow::json::rvalue

[gittiver]

I will not add the returnable subclass to the library,
you can use response (std::string contentType, std::string body_) instead.

return response("json", your_body_string) should work.

[CarloDG80]

I will not add the returnable subclass to the library, you can use response (std::string contentType, std::string body_) instead.

return response("json", your_body_string) should work.

Super,
Thank you

[CarloDG80]

Would you like something like this?

static inline crow::response makeJsonResp(std::string jsonData) {
    return crow::response("json", jsonData );
}