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')

and actually, that stream= argument defaults to 'stdout' if it isn't given.

I modified a test that will break if this functionality is ever removed.