Allow callable in configuration for plugin.type

Example:

```python3
\# Load default configuration
my_config = config.load()

\# Pass a class directly
my_config.update({"auth": {"type": MyAuth}})

\# Pass an object directly
my_rights = MyRights()
my_config.update({"rights": {"type": lambda config: my_rights}})

app = Application(my_config)
````

radicale/utils.py

@@ -23,6 +23,9 @@ from radicale.log import logger
 
 def load_plugin(internal_types, module_name, class_name, configuration):
     type_ = configuration.get(module_name, "type")
+    if callable(type_):
+        logger.info("%s type is %r", module_name, type_)
+        return type_(configuration)
     if type_ in internal_types:
         module = "radicale.%s.%s" % (module_name, type_)
     else: