add support & colour tags to letter_box (#1005)

Author: mnalis

data/presets/amenity/letter_box.json

@@ -6,6 +6,7 @@
         "height"
     ],
     "moreFields": [
+        "colour",
         "covered",
         "indoor",
         "level",
@@ -14,6 +15,7 @@
         "manufacturer",
         "material",
         "operator",
+        "support",
         "wheelchair"
     ],
     "geometry": [