Add ability to use multple geocoder sources

[sdiltsDNRC]

We have a map that needs to pull from multiple geocoder services to get the results we need. We are specifically pulling from ArcGis geocoder servers, but baking this into the generic interface work would well.

[sdiltsDNRC]

I can think of two generic ways to do this:

1. Add a new geocoder interface that merges results from different geocoders
2. Take a list or a single geocoder when constructing the widget.

Option 1 sounds less disruptive and a bit easier, so I'll give that a shot.