add relaxation flag to validator call

[mpacer]

This arises in the context of determining what sorts of intermediate representations we want for notebooks in nbconvert: jupyter/nbconvert#643

It was suggested by @minrk that we only allow additional fields (and no additional values for existing fields or the deletion of fields) in terms of this intermediate representation.

Up until now, we thought that preprocessors had always only created valid notebooks. That turns out to be not be the case (jupyter/nbconvert#645).

It would be nice to be able to establish that the only things allowed by preprocessors is to exactly what we say it is.

To that end, this adds a flag to the validate API (and the get_validator API) for communicating that that specific relaxation is to be adhered to and otherwise the notebook is entirely valid.

This allows the use of nbformat.validate(…, relax_add_props=True) in the nbconvert preprocessor checks which will ensure that 3rd party preprocessors similarly adhere to this restriction on the nbconvert internal representation.