add cell-level metadata

[minrk]

We have had various proposals of ideas for features that require cell-level metadata. We should make sure we iron out a mechanism by which these can be added prior to 0.13 release, so we reduce the likelihood of needing to rev the nbformat for 0.14. I imagine simply attaching a simple black-box metadata dict to each cell (and probably worksheet) like is already on the notebook as a whole would be sufficient. They can obviously start out as empty since we don't have any content yet.

Related: I believe the recent js merge removed 'collapsed' from the saved data (It's started disappearing when I re-save notebooks). If we do want to keep storing collapsed, it probably belongs in this md dict.

[fperez]

If we do this, we should also add a metadata dict to each worksheet as well. Right now worsheets are just kept in a list, and I imagine once we have UI for them (even if it doesn't happen for 0.13), people will immediately want to, at the very list, name them. And I can immediately think of other things to do with sheet-level metadata.

It would also give us consistently metadata support at the various levels of our hierarchy: notebook, worksheet, cell. It seems that would lead to a nice, uniform model.

[minrk]

Yes, absolutely - I thought I had typed that. I definitely thought it.

[fperez]

We're all really needing telepathic email/composition, I'm telling you ;)

[takluyver]

@fperez : I expect we're all thinking that ;-)