Store "displayable" cells in a DataFrame in `read_notebook`

[choldgraf]

As I was running notebooks with papermill, I found myself intuitively expecting that some kind of attribute would be added to the notebook telling me which of the cells were "displayable". Basically, something that'd tell me which keys were valid for .display_output(key). Is there a way to retrieve such a list from a notebook that has been run, or do you need to know the name of the output ahead of time?

[MSeal]

The function _get_notebook_outputs does this. We should probably expose it on the notebook object there. I think we might be moving a lot of this logic out of papermill and into a scrapbook project moving forward. See https://github.com/nteract/papermill/issues/175