add metadata to nbformat Python API

IPython/nbformat/v3/nbbase.py

@@ -53,12 +53,18 b' def from_dict(d):'
 def new_output(output_type=None, output_text=None, output_png=None,
     output_html=None, output_svg=None, output_latex=None, output_json=None,
     output_javascript=None, output_jpeg=None, prompt_number=None,
-    ename=None, evalue=None, traceback=None, stream=None):
+    ename=None, evalue=None, traceback=None, stream=None, metadata=None):
     """Create a new code cell with input and output"""
     output = NotebookNode()
     if output_type is not None:
         output.output_type = unicode(output_type)
 
+    if metadata is None:
+        metadata = {}
+    if not isinstance(metadata, dict):
+        raise TypeError("metadata must be dict")
+    output.metadata = metadata
+
     if output_type != 'pyerr':
         if output_text is not None:
             output.text = unicode(output_text)
@@ -91,7 +97,7 b' def new_output(output_type=None, output_text=None, output_png=None,'
 
     if output_type == u'stream':
         output.stream = 'stdout' if stream is None else unicode(stream)
-
+    
     return output