Support multiple databases. (#68)

conftest.py

@@ -13,10 +13,15 @@ from aerich.ddl.sqlite import SqliteDDL
 from aerich.migrate import Migrate
 
 db_url = os.getenv("TEST_DB", "sqlite://:memory:")
+db_url_second = os.getenv("TEST_DB_SECOND", "sqlite://:memory:")
 tortoise_orm = {
-    "connections": {"default": expand_db_url(db_url, True)},
+    "connections": {
+        "default": expand_db_url(db_url, True),
+        "second": expand_db_url(db_url_second, True),
+    },
     "apps": {
         "models": {"models": ["tests.models", "aerich.models"], "default_connection": "default"},
+        "models_second": {"models": ["tests.models_second"], "default_connection": "second"},
     },
 }