allow Viewpoints to be mapped as areas, closes #650

Author: tyrasd

data/presets/tourism/viewpoint.json

@@ -2,7 +2,8 @@
     "icon": "temaki-spotting_scope",
     "geometry": [
         "point",
-        "vertex"
+        "vertex",
+        "area"
     ],
     "fields": [
         "direction"