Feature-specific zoom calculations

[mapsam]

This PR adds some extra logic that allows us to begin calculating better maximum zooms based on feature types rather than just file size and extent using the mapnik.datasource.describe() method, can tell us if we are working with point, line, polygon, or mixed datasets. Right now this only adds a newly calculated "smallest maximum zoom" for point datasets that are less than 1MB.

In the future, we should look into:

• returning different calculations based on file size (instead of a hard break at 1MB)
• calculating better zooms for lines and polygons (need to investigate the best way to calculate a real data-density ratio)

refs: https://github.com/mapbox/mapnik-omnivore/issues/140

cc @GretaCB @who8mycakes

[GretaCB]

Love iiiiiit 🙌 Only 👀 are 🔴 Travis tests.

[coveralls]

Coverage increased (+0.1%) to 95.212% when pulling 532a0f0e3a76500c441786e93a559d7551cb2076 on we-need-to-go-deeper into a43406828db595486db2a7ebaebdf8d941a793a1 on 8.3.0.