Accumulate distances by road class

[robfitzgerald]

In order to quantify energy savings by trip type, we should know what trips cover what amount of highway, arterial, and other road types, so that in our summarization, we might have the following:

"road_class_distance": {
  "highway": 0.01,
  "motorway": 0.5,
  "residential": 2.352
}

which tells us that this trip covers mostly residential streets.

this should be accomplished with an output plugin instead of appending more state to the search.