http_response.h : added custom type support

[hg333]

Added a response constructor where content-type can be specified by the user
Usage

    CROW_ROUTE(app,"/test")([](){
        std::ostringstream os;
        os<<"<First>";
        os<<"<Content>";
        os<<"Element1";
        os<<"</Content>";
        os<<"<Content>";
        os<<"Element2";
        os<<"</Content>";
        os<<"<Content>";
            os<<"<InnerContent>";
            os<<"Element3";
            os<<"</InnerContent>";
        os<<"</Content>";
        os<<"</First>";
        
        return crow::response("xml",os.str());
    });

This PR aims to fix: #48

[The-EDev]

Thanks for your work, will merge as soon as CI is done