Improve the speed of valdiation (#1672)

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:
  - #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.

nbconvert/exporters/exporter.py

@@ -61,6 +61,10 @@ class Exporter(LoggingConfigurable):
         help="Extension of the file that should be written to disk"
     ).tag(config=True)
 
+    optimistic_validation = Bool(False,
+        help = "Reduces the number of validation steps so that it only occurs after all preprocesors have run."
+    ).tag(config=True)
+
     # MIME type of the result file, for HTTP response headers.
     # This is *not* a traitlet, because we want to be able to access it from
     # the class, not just on instances.
@@ -296,6 +300,13 @@ def _init_resources(self, resources):
         resources['output_extension'] = self.file_extension
         return resources
 
+    def _validate_preprocessor(self, nbc, preprocessor):
+        try:
+            nbformat.validate(nbc, relax_add_props=True)
+        except nbformat.ValidationError:
+            self.log.error('Notebook is invalid after preprocessor %s',
+                               preprocessor)
+            raise 
 
     def _preprocess(self, nb, resources):
         """
@@ -321,11 +332,10 @@ def _preprocess(self, nb, resources):
         # to each preprocessor
         for preprocessor in self._preprocessors:
             nbc, resc = preprocessor(nbc, resc)
-            try:
-                nbformat.validate(nbc, relax_add_props=True)
-            except nbformat.ValidationError:
-                self.log.error('Notebook is invalid after preprocessor %s',
-                               preprocessor)
-                raise
+            if not self.optimistic_validation:
+                self._validate_preprocessor(nbc, preprocessor)
+
+        if self.optimistic_validation:
+          self._validate_preprocessor(nbc, preprocessor) 
 
         return nbc, resc