[geoip2-golang]: expose maxminddb reader

[tehnerd]

sometimes it is usefull to have an access to raw maxminddb reader ( e.g. to use Networks feature; to iterate over db and build a reverse one ("show me all ips which belongs to XXx")). it feels like adding such feature to raw geoip library is to increase a complexity for not so common usecase; hence just exposing Reader on demand and allow for ppl who needs it to use it directly is cleaner method as for now