ipywidgets should implement _repr_mimebundle

[joequant]

This is an issue related to Calysto/metakernel#207

Right now ipywidgets does not work with metakernel. There is a patch in Calysto/metakernel#207 that fixes the problem but it involves copying code that is in _ipython_display(). Would be useful to create a _repr_mimebundle that generates the mime for a widget so that metakernel can process it through its own display system.

[jasongrout]

We implement _repr_mimebundle in master (see #2021), which hasn't been released yet in ipywidgets 8.