fix: let config class specify default app handler, engine maker

this avoids the need for a config subclass to use `setdefault()` hacks
to specify default app handler for instance, since that approach must
compete with config extensions who also may wish to do that.

similar concept for the engine maker; notably the rattail project
needs to override this function somewhat and we need a way to allow
for that without (re-)introducing the app handler here.

tests/test_conf.py

@@ -10,6 +10,7 @@ import sqlalchemy as sa
 from wuttjamaican import conf
 from wuttjamaican.exc import ConfigurationError
 from wuttjamaican.db import Session
+from wuttjamaican.db.conf import make_engine_from_config
 from wuttjamaican.app import AppHandler
 from wuttjamaican.testing import FileConfigTestCase
 
@@ -386,12 +387,43 @@ configure_logging = true
         self.assertEqual(config.get_list('foo.bar'), ['hello', 'world'])
 
     def test_get_app(self):
+
+        # default handler
         config = conf.WuttaConfig()
+        self.assertEqual(config.default_app_handler_spec, 'wuttjamaican.app:AppHandler')
         app = config.get_app()
-        # make sure we get the true default handler class
         self.assertIsInstance(app, AppHandler)
+        # nb. make extra sure we didn't get a subclass
         self.assertIs(type(app), AppHandler)
 
+        # custom default handler
+        config = conf.WuttaConfig()
+        config.default_app_handler_spec = 'tests.test_conf:CustomAppHandler'
+        app = config.get_app()
+        self.assertIsInstance(app, CustomAppHandler)
+
+    def test_get_engine_maker(self):
+
+        # default func
+        config = conf.WuttaConfig()
+        self.assertEqual(config.default_engine_maker_spec, 'wuttjamaican.db.conf:make_engine_from_config')
+        make_engine = config.get_engine_maker()
+        self.assertIs(make_engine, make_engine_from_config)
+
+        # custom default func
+        config = conf.WuttaConfig()
+        config.default_engine_maker_spec = 'tests.test_conf:custom_make_engine_from_config'
+        make_engine = config.get_engine_maker()
+        self.assertIs(make_engine, custom_make_engine_from_config)
+
+
+class CustomAppHandler(AppHandler):
+    pass
+
+
+def custom_make_engine_from_config(*args, **kwargs):
+    pass
+
 
 class TestWuttaConfigExtension(TestCase):