Add config to only show largest buildings early before zoom 16

[nvkelso]

The tile size for 512px sized zoom 13 and 14 tiles is large because we're including all the buildings instead of just some of the largest buildings. At earlier zooms we filter by area more aggressively.

Some map styles (like those produced at Mapzen) assume 100% building coverage at those zooms, so let's add a config to adjust the area threshold to show "all" or just "some" buildings at the earlier zooms. By zoom 15 (512px size, or 16 in 256px space) all the buildings should remain visible. The default config should be "all" not "some" for a compatible v1.9 change, or "some" for a breaking v2 change.

Example 512 px tiles: 13/4149/2817.mvt and 14/8174/6234.mvt