VT3 support

[joto]

This ticket tracks the development of support for the VT3 format. (https://github.com/mapbox/vector-tile-spec/tree/v3.0-development).

Branch vt3

Contains support for:

• string ids
• inline attributes including new array and map nested attributes (Complex Properties Overhaul vector-tile-spec#121)
• number lists
• 3d geometry support
• x/y/zoom in layer
• geometric attributes
• spline support

@ericfischer @flippmoke