Add aggregating geocoder

[sdiltsDNRC]

This adds the AggregatingGeocoder class, which combines the results of several geocoders together.

Questions / Observations:

• I couldn't think of a good name for this geocoder, I'm happy to update it to something better.
• What documentation should I provide/
• Should the default options argument have a default geocoder?

I'll add actual tests if this looks like a good solution.

See #368.