Add Maxminddb support

Geolite2 datasets are also available in a custom binary format described [here](http://maxmind.github.io/MaxMind-DB/). Might be worth implementing a reader as it is probably more efficient than using the csv version of the database. There is a lot of prior art (much due to the authors of the specification itself) in other languages we can use as reference:

- [Go package](https://github.com/oschwald/geoip2-golang)
- [Python package](https://github.com/maxmind/GeoIP2-python)
- [C library](https://github.com/maxmind/libmaxminddb)