Added a new geoip filter

Hi,

I've created a geoip filter for node-logstash. haven't tested it yet, but should work.

It uses the geoip-lite (`npm install geoip-lite`) library, from https://github.com/bluesmoon/node-geoip
and is able to lookup an ip's location and could add the following data:
 - *_geoip_country: country code
 - *_geoip_region: the region
 - *_geoip_city: the city
 - *_geoip_lat: the lat
 - *_geoip_lon: the lon

 the asterisk'es are replaced with the attribute defined by `use_attr``(defaults to host).

Usage `filter://geoip://?use_attr=postfix_source&save_city=false&save_region=false&save_latlon=false&save_country=true`