Support encoding and decoding lists

[zerebubuth]

MVT doesn't have a native list value type, but we can fake it by encoding lists to something like "list:" + value.to_json, and parse it from JSON when the prefix is list: when decoding. This runs the risk of decoding things which start with list: which aren't actually lists, but I think there are few enough of them?

[nvkelso]

Apparently Mapbox encodes raw JSON for nested properties (e.g.: for their file uploader > tiles tool).

[nyurik]

I just made an implementation proposal for objects and lists at mapbox/vector-tile-spec#75 (comment)