ipython/ipython — pull request #6991 by minrk (2014-11-20 00:27:41+00:00)
Title: Handle unrecognized output and cell types

Makes adding new cell and output types a minor revision, rather than a major one.

A notebook with minor nbformat from the future will pass validation if it has cells or outputs that are unrecognized.

These cells and outputs are displayed with simple red-text placeholders. Their content is not touched nor displayed at all.