Add a property to nbformat for storing multimedia

[adpatter]

This issue is related to The browser hangs when a Notebook is opened that contains hundreds of MB of data in the metadata property.

We have a use case where we want to add audio and video to a Notebook that is not associated with any one cell. These are my thoughts:

1. This isn't metadata; hence, it shouldn't be stored in the metadata property.
2. This kind of content probably shouldn't be editable; hence, it shouldn't show up in the metadata editor.
3. Loading the Notebook shouldn't be contingent on processing this data. Presently, if this data is stored in the Notebook metadata it is handled the same way that other metadata is handled, which is probably contributing to the issue described in 12145.
4. Such a feature may facilitate new innovative ways of using Notebooks.