Added flags that allow ipv6 databases to be used

I've added the flags that make the plugin support ipv6 city and country databases.

I was unable to get the rspec test suite running (even in pristine mode), probably due to the separate repository setup, but I've verified that this works using a simple test config and the getting started setup. Test databases were retrieved from maxmind using the `geoip-database-contrib` package in ubuntu trusty (multiverse).