Add dependency alias support

This commit allows to specify an arbitrary number of aliases for a
dependency. A project can be generated using that dependency either
referring to its main id or any of its registered aliases.

Fixes gh-29

initializr-service/application.yml

@@ -20,6 +20,8 @@ initializr:
           id: jdbc
         - name: JPA
           id: data-jpa
+          aliases:
+            - jpa
         - name: MongoDB
           id: data-mongodb
         - name: Redis