Add support for skipping cells

[ianhellstrom]

Notebooks often include cells that are mostly exploratory, e.g. looking at a histogram and then making a decision on how to proceed with the code (e.g. decide on a cut-off value). Re-running the charts may not make sense in a non-interactive way.

It would therefore be great if Papermill supported a cell tag (e.g. "skip") for cells that are not executed when running with Papermill.

[MSeal]

This is duplicated by #429 -- if you wouldn't mind scrolling the the bottom of that and commenting on the two proposed paths forward we'll likely implement one or both of those in the nearish future.