Added PillarExtension.setup_app(app)

It's called on each extension after all extensions have been processed,
and after all built-in Pillar modules have had their setup_app() called.
Call order is random.

pillar/extension.py

@@ -62,3 +62,6 @@ class PillarExtension(object):
 
         :rtype: dict
         """
+
+    def setup_app(self, app):
+        """Called during app startup, after all extensions have loaded."""