Add a plugin infrastructure

jedi/plugins/base.py

@@ -0,0 +1,10 @@
+class BasePlugin(object):
+    """
+    Plugins are created each time an evaluator is created.
+    """
+    def __init__(self, evaluator):
+        # In __init__ you can do some caching.
+        self._evaluator = evaluator
+
+    def execute(self, default_callback, arguments):
+        return default_callback(arguments)