Support for GeoHash grid in geo-location plugin

Feature Request

## Expected Behavior
Have geohash fields instead of strings.

## Current Behavior
Graylog insert geo-location data as string into Elasticsearch.

## Possible Solution
Insert geo-location data as [GeoHash](https://www.elastic.co/guide/en/elasticsearch/reference/current/search-aggregations-bucket-geohashgrid-aggregation.html)

## Steps to Reproduce (for bugs)
1) Enable the geo-location plugin
2) Use it :)

## Context
Elasticsearch supports GeoHash grid fields, whats can be used in many other applications such as Grafana to plot geolocation data into panels.
Would will be really good if Graylog could insert geo-location data into ES using a supported and standard type instead of just a string field. This would open the doors for more integrations.

## Your Environment
* Graylog Version: 3.0.2
* Elasticsearch Version: 6.0
* MongoDB Version: 3.2
* Operating System: Debian Stretch 9.0
* Grafana: 6.3.3 [Worldmap plugin](https://github.com/grafana/worldmap-panel)