Broken compability with mapbox/geojson-cpp

[mathisloge]

With the change from recursive_wrapper to shared_pointers in #68 the compability with https://github.com/mapbox/geojson-cpp is not given anymore.

I needed to inject this:

    rapidjson_value operator()(const std::shared_ptr<std::unordered_map<std::string, value>>& map) {
        return (*this)(*map);
    }

    rapidjson_value operator()(const std::shared_ptr<std::vector<value>>& map) {
        return (*this)(*map);
    }

into https://github.com/mapbox/geojson-cpp/blob/master/include/mapbox/geojson_impl.hpp