Add `_distance` measure to response data

[jboarman]

It would be helpful to consumers if the data returned by Orbitron includes a distance measure. Since every query requires a given data point (whether it be a specific lat/long or zip centroid), then we should be able to calculate a distance between the input and each row of the output.

At some point, we may need to respond with varying units (miles vs km), or provide code for helper functions that convert between units, or allow parameters to be passed that dictate units to include in the response. However, the current project only supports miles at the moment. For this issue, perhaps we just add the units to the response, like _distance: "1.5 mi"? (The underscore is added here to avoid clashing with the source's data model.)