Add support for configurable pixel sizes

[rmarianski]

Connects to #810

Included in this pr is a system to assign classes to features. Based on earlier conversations, I assume that we want to expose these properties in the tiles themselves. If we don't, we should rename to mz_class and they will get dropped. I also just made them integers for now; but they can be changed to whatever is easier to work with. Something else to consider is that the word class might not be the smartest choice because it's a keyword in many languages. However, I tried it in a node.js repl and didn't seem to have any problems with it though.

The implementation here re-uses the csv machinery to specify and assign these properties. The idea is that the mz_min_pixels spreadsheets will define the rules for the configurable pixel sizes, and a transform is configured that drops any features with areas that don't meet this threshold. The transform assumes that the thresholds will be in pixel values, and calculates the required area for the feature based on the zoom level. Having the capability to assign class values could be useful to determine the pixel thresholds.

Note that there aren't any rules specified, but the pieces are in place to honor any subsequent changes.

@zerebubuth, @nvkelso could you review please?

[zerebubuth]

Looks good to me! 👍