Add config extension class, tests, docs

docs/narr/config/object.rst

@@ -54,3 +54,42 @@ to solve the "chicken-vs-egg" problem::
    config = make_config()
 
    app = config.get_app()
+
+
+Extending the Config Object
+---------------------------
+
+Some packages may need to "extend" the config object somehow, to add
+various attributes which they may reference later when certain code
+runs.
+
+A typical example would be for an "integration" package, which is
+responsible for communicating with a third party database.  This
+package might extend the config by loading some database connections
+based on config values, and attaching those directly to the config
+object (and usually, configuring a ``Session`` class).
+
+But here is a simpler example; add this to e.g. ``poser/config.py``::
+
+   from wuttjamaican.conf import WuttaConfigExtension
+
+   class PoserConfigExtension(WuttaConfigExtension):
+       """
+       Custom config extension for Poser
+       """
+       key = 'poser'
+
+       def configure(self, config):
+           foo = config.setdefault('poser.foo', 'bar')
+           config.poser_foo = foo
+
+Then you must register an :term:`entry point` in your ``setup.cfg``:
+
+.. code-block:: ini
+
+   [options.entry_points]
+   wutta.config.extensions =
+        poser = poser.config:PoserConfigExtension
+
+After your ``poser`` package is installed, the extension logic should
+automatically run when the config object is being made.