Preserve feature identifiers

[1ec5]

convert() includes the geometry and properties of each feature in the given feature collection, but it throws away the feature’s ID. This means any filter in mapbox-gl-native that depends on the existence of an $id property in a GeoJSON source will behave incorrectly.

/ref mapbox/vector-tile-js#46, mapbox/mapbox-gl-native#5708
/cc @jfirebaugh @ivovandongen @cammace