Add support for extensions

This is experimental.

extensions/example/script.py

@@ -0,0 +1,14 @@
+def input_modifier(string):
+    """
+    This function is applied to your text inputs before
+    they are fed into the model.
+    """ 
+
+    return string.replace(' ', '#')
+
+def output_modifier(string):
+    """
+    This function is applied to the model outputs.
+    """
+
+    return string.replace(' ', '_')