Merge pull request #553 from mapbox/zoom-filter

Allow filtering features by zoom level in conditional expressions

tests/feature-filter/places-filter

@@ -0,0 +1,12 @@
+{ "*": [
+	"any",
+	[ "all", [ "==", "SCALERANK", 0 ], [ ">=", "$zoom", 0 ] ],
+	[ "all", [ "==", "SCALERANK", 1 ], [ ">=", "$zoom", 1 ] ],
+	[ "all", [ "==", "SCALERANK", 2 ], [ ">=", "$zoom", 2 ] ],
+	[ "all", [ "==", "SCALERANK", 3 ], [ ">=", "$zoom", 3 ] ],
+	[ "all", [ "==", "SCALERANK", 4 ], [ ">=", "$zoom", 4 ] ],
+	[ "all", [ "==", "SCALERANK", 5 ], [ ">=", "$zoom", 5 ] ],
+	[ "all", [ "==", "SCALERANK", 6 ], [ ">=", "$zoom", 6 ] ],
+	[ "all", [ "==", "SCALERANK", 7 ], [ ">=", "$zoom", 7 ] ],
+	[ "all", [ ">=", "SCALERANK", 8 ], [ ">=", "$zoom", 8 ] ]
+] }