create an activatable transformer classes

those are no-op by default

converters/transformers.py

@@ -29,12 +29,15 b' class ConfigurableTransformers(Configurable):'
         raise NotImplementedError('should be implemented by subclass')
 
 
-class Foobar(ConfigurableTransformers):
-    message = Unicode('-- nothing', config=True)
+class ActivatableTransformer(ConfigurableTransformers):
 
+    active = Bool(False, config=True)
 
-    def cell_transform(self, cell, other, index):
-        return cell, other
+    def __call__(self, nb, other):
+        if not self.active :
+            return nb,other
+        else :
+            return super(ActivatableTransformer,self).__call__(nb, other)
 
 
 def cell_preprocessor(function):
@@ -73,7 +76,7 b' def haspyout_transformer(cell, other, count):'
 
 # todo, make the key part configurable.
 
-class ExtractFigureTransformer(ConfigurableTransformers):
+class ExtractFigureTransformer(ActivatableTransformer):
     enabled = Bool(False,
             config=True,
             help=""" If set to false, this transformer will be no-op """

runme.py

@@ -30,7 +30,7 b' from IPython.config.application import Application'
 from IPython.config.loader import ConfigFileNotFound
 from IPython.utils.traitlets import List, Unicode, Type, Bool, Dict, CaselessStrEnum
 
-from converters.transformers import (ConfigurableTransformers,Foobar,ExtractFigureTransformer)
+from converters.transformers import (ConfigurableTransformers,ExtractFigureTransformer)
 
 
 class NbconvertApp(Application):
@@ -58,7 +58,6 b' class NbconvertApp(Application):'
         self.classes.insert(0,ConverterTemplate)
         # register class here to have help with help all
         self.classes.insert(0,ExtractFigureTransformer)
-        self.classes.insert(0,Foobar)
         # ensure those are registerd
 
     def load_config_file(self, profile_name):