Permit multiple buckets in coordinate map visualization

[sempervictus]

Is your feature request related to a problem? Please describe.

The current implementation of the geopoint visualization only permits rendering a single geo field in the selected index. Situations where both client and server are geocoded, or where a traceroute needs to be visualized, require the ability to have multiple points rendered on a single visualization.

Describe the solution you'd like

Permit multiple buckets visualizing the points in an index.

[joshuarrrr]

The existing mapping functionality in Dashboards is a bit limited. For more complex use cases, you may have better luck with the dedicated mapping plugin: #15