Minfy JSON (filter out comments and son) before deserialisation

The JSON parser allows specifying a callback for this purpose:

    /** [...]
    @param[in] i  input to read from
    @param[in] cb  a parser callback function of type @ref parser_callback_t
    which is used to control the deserialization by filtering unwanted values
    (optional)
    */
    static basic_json parse(detail::input_adapter&& i,
                            const parser_callback_t cb = nullptr,
                            const bool allow_exceptions = true)