Merge pull request #1870 from minrk/captureio New `%%capture` cell magic captures stdout/err while running a cell. Uses `capture_output()` context manager, moved to utils.io from IPython.parallel testing utilities, where it originated. The caputre objects can be printed as a string, case in which they display the captured stdout, which is also available as `.stdout`. The captured stderr, if any, is in a `.stderr` attribute. A `.show()` method can be called to quickly print both, with stderr being correctly printed to the sys.stderr stream (so the notebook displays it with red highlighting). closes #1863