Optionally speed up validation

[gwincr11]

Motivation:

• We are looking to build out a notebook viewer with an eye toward performance.
  In our flamegraph research we found that as much as 90% of the time spent in
  Notebook creation can occur in the preprocessor valdiation logic. This pr
  introduces the optional idea of optimistic validation, in which we care not
  about which preprocessor introduces a validation error but that the notebook
  ends in a valid state, since we care more about speed.

Related Issues:

• Performance: skip repeated validator runs between preprocessors #1663

Changes:

• Introduces a config option to remove some of the validation checks.

Questions:

• I am new to the project and am not sure where a test should live for
  this logic, or the best way to structure such a test. If a reference
  is available I will happily added a test.

[MSeal]

Thanks for making a PR for the change. There isn't a great example to copy for writing a test here. I'm good with merging as is but if you would like to add a test after you'd need to mock the validate call, trigger a preprocess event with the flag on, and then count the number of calls made.

[gwincr11]

Thanks for the merge @MSeal I was less wondering how to test it and more wonder where it should live and how to setup the test properly, how to call nbconvert and pass it a file for example and what file this test best fits into. Thanks again!