Adding a new 'load_callback_plugins' config option, defaults to False

This option, when set to True, allows "ansible" command to load callback plugins

bin/ansible

@@ -38,6 +38,8 @@ class Cli(object):
     def __init__(self):
         self.stats = callbacks.AggregateStats()
         self.callbacks = callbacks.CliRunnerCallbacks()
+        if C.DEFAULT_LOAD_CALLBACK_PLUGINS:
+            callbacks.load_callback_plugins()
 
     # ----------------------------------------------