For label placements, enable configurable pixel size threshold

[nvkelso]

The label generation logic should probably have a separately configurable pixel size threshold generically per layer, and specifically per feature selection in the YAML.

For instance, if landuse layer polygons are 1 pixel threshold, then we might want 10 pixel threshold for label generation in general (queries.yaml), but maybe it should be different for national parks only (either in landuse.yaml or in the TileStache transform?).

Connects to: #202

[nvkelso]

Connects to #583.

[nvkelso]

Talked a bit about this IRL, assigning to @rmarianski.

[nvkelso]

If further work is needed here, we'll make a new issue in a new milestone.