Add support for configuration extensions

Allow blocks prefixed with `x-` in the configuration as a place to
declare reusable blocks with YAML anchors and aliases.

Borrowed from the Docker Compose configuration file format -
https://github.com/compose-spec/compose-spec/blob/main/spec.md#extension

Thanks to @ruyrocha for the suggestion.

test/fixtures/deploy_with_extensions.yml

@@ -0,0 +1,24 @@
+
+x-web: &web
+  traefik: true
+
+service: app
+image: dhh/app
+servers:
+  web_chicago:
+    <<: *web
+    hosts:
+      - 1.1.1.1
+      - 1.1.1.2
+  web_tokyo:
+    <<: *web
+    hosts:
+      - 1.1.1.3
+      - 1.1.1.4
+env:
+  REDIS_URL: redis://x/y
+registry:
+  server: registry.digitalocean.com
+  username: user
+  password: pw
+primary_role: web_tokyo