Visualize Differences in Fixture Tests

[flippmoke]

We had the start of some work towards using https://github.com/mapnik/geometry-test-data/ for fixture testing as part of Wagyu and this is excellent! However, it is very difficult to see differences in the results from wagyu currently as we can not visually inspect the differences quickly. Additionally the input and output formats are not in a format that can be easily loaded into other software. Perhaps updating this to be geojson or another format?

/cc @tcql @jakepruitt