adding stream kwarg to current.new_output

[ivanov]

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.

[minrk]

Is there anywhere in our code that was using the clunky path that should be cleaned up?

[ivanov]

nope, just ran grep -R new_output * | cut -f 1 -d: | sort | uniq and checked all of those. Should I bother applying the same change to nbformat/v2/nbbase.py?

[minrk]

No, v2 should not be touched - it should be frozen. I'll go ahead and merge, then. Thanks!