Allow outbox box #s to be links, instead of "labels"

[ihodes]

i.e. ?hyper=Out[2]

[tavinathanson]

@ihodes some reasons I didn't start off basing the logic on cell numbers was:

• Re-running notebooks generates different cell numbers.
• Notebook specific vs. general case.

But probably worth doing anyway?

[ihodes]

Eh, #1 in particular is a very good point. Not sure you want to open that can of worms by supporting this. I'd close this, but up to you.

[ihodes]

Wish there were a better solution than adding some new syntax into a notebook, though…

[tavinathanson]

Actually don't mind adding the syntax, since it's unrelated to the notebook and the cell: allows pulling "labels" out of cells that contain other stuff.

Supporting cell numbers seems somewhat reasonable if the user plans to restart and re-run the entire notebook, which should re-number the cells in the identical way, and only append cells vs. re-ordering, resulting in some sort of "true" cell number. Will leave this open for now as I/we play around with this.

[tavinathanson]

@ihodes another Jupyter-specific approach could be to find matching In[2] and Out[2] cells, and use the contents of In[2] as the label for Out[2]. Would only support the use case of outputting single variable contents directly to a cell vs. outputting various things to a cell, but still possibly useful.