Add ISO 3166-1 alpha-2 country codes as iso_a2 attribute

Fix #306

layers/place/place.yaml

@@ -34,6 +34,11 @@ layer:
       - suburb
       - neighbourhood
       - isolated_dwelling
+    iso_a2:
+      description: |
+          Two-letter country code [ISO 3166-1 alpha-2](https://en.wikipedia.org/wiki/ISO_3166-1_alpha-2).
+          Original value of the
+          [`country_code_iso3166_1_alpha_2`](http://wiki.openstreetmap.org/wiki/Tag:place%3Dcountry) tag.
     rank:
       description: |
           Countries, states and the most important cities all have a
@@ -53,7 +58,7 @@ layer:
   buffer_size: 256
   datasource:
     geometry_field: geometry
-    query: (SELECT geometry, name, name_en, name_de, {name_languages}, class, rank, capital FROM layer_place(!bbox!, z(!scale_denominator!), !pixel_width!)) AS t
+    query: (SELECT geometry, name, name_en, name_de, {name_languages}, class, rank, capital, iso_a2 FROM layer_place(!bbox!, z(!scale_denominator!), !pixel_width!)) AS t
 schema:
   - ./types.sql
   - ./capital.sql