adding stream kwarg to current.new_output

This was missing, and made unnecessarily clunky to create output cells
of stream type using the nbformat API.

Before this commit, you had to do something like

  from IPython.nbformat import current as c
  output = c.new_output('stream', the_text)
  output['stream'] = 'stdout'

after this commit

  from IPython.nbformat import current as c
  output = c.new_output('stream', the_text, stream='stdout')

IPython/nbformat/v3/nbbase.py

@@ -53,7 +53,7 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,
-    etype=None, evalue=None, traceback=None):
+    etype=None, evalue=None, traceback=None, stream=None):
     """Create a new code cell with input and output"""
     output = NotebookNode()
     if output_type is not None:
@@ -89,6 +89,9 b' def new_output(output_type=None, output_text=None, output_png=None,'
         if traceback is not None:
             output.traceback = [unicode(frame) for frame in list(traceback)]
 
+    if output_type == u'stream':
+        output.stream = 'stdout' if stream is None else unicode(stream)
+
     return output
 
 

IPython/nbformat/v3/tests/nbexamples.py

@@ -93,6 +93,7 b' ws.cells.append(new_code_cell('
         output_text='foo\rbar\r\n'
     ),new_output(
         output_type=u'stream',
+        stream='stderr',
         output_text='\rfoo\rbar\n'
     )]
 ))