only use fenced code for execution

execute.py

@@ -9,7 +9,7 @@ from traitlets.config import Config
 from nbconvert_fix import ExtractOutputPreprocessor
 
 
-reader = notedown.MarkdownReader()
+reader = notedown.MarkdownReader(code_regex='fenced')
 
 mimetypes.add_type('application/vnd.plotly.v1+json', '.json')