Add support for "mapnik-geometry-type" in filters

See https://github.com/mapbox/carto/issues/180

lib/carto/tree/filter.js

@@ -1,6 +1,13 @@
 (function(tree) {
 
 tree.Filter = function Filter(key, op, val, index, filename) {
+
+    // See https://github.com/mapbox/carto/issues/180
+    // TODO: use mapnik-reference for these conversion ?
+    if ( key == "mapnik-geometry-type" ) {
+      key = "mapnik::geometry_type"
+    }
+
     if (key.is) {
         this.key = key.value;
         this._key = key;