Merciful validation

This is related to #235 but provides for an alternative path. There are instances when users would *want* to fixup the notebook, so we should provide a way for them to do so.

Right now, if you have a validation error for non-unique cells, you will get a validation error informing you that a cell had a duplicate cell_id, and also the notebook that you passed in will be modified with *only* the first instance of a non-unique cell. I agree with the approach @Carreau outlines in #235, but want it to be possible to *opt-in* to fixing the validation errors that are fixable (such as replacing all non-unique cell ids with unique ones). I think it would makes sense to do this via a new `repair` kwarg that would default to `False`, and keep with either current behavior, or non-modifying behavior as per #235, but when `repair=True` we would fix up the errors and do a best effort of massaging the notebook into one that is compliant with a given nbformat version.