Add support for complex properties from MVT formats

[nvkelso]

Similar to #1617 and related to the root cause of #1616.

I've split off here MVT here from GeoJSON and TopoJSON as the Tilezen MVT format currently drops properties with lists (or other nesting) no the server during the format conversion.

The sister issue over in Tilezen is tilezen/mapbox-vector-tile#94 to add stringified MVT properties.

[matteblair]

Tangram JS is adding support for structured property data in the form of stringified JSON: tangrams/tangram#715

Tangram ES will add matching support. This involves several both parsing string properties as JSON where appropriate and changing our feature data model to accommodate list and mapping properties in addition to scalar values.