Add ExcludeUnlessExplicit as a config option for Versioning bundle

The use case for this is having a document with some changes that need to be preserved and others that really don't.  For example I want to keep track of a Product's changes so that we can see exactly what it looked like when it was ordered.  However the product also has stock quantity in it and we really don't want to keep a revision every time the stock goes up or down. 

So the idea is you add the metadata key Version-This-Save into the document in the session.  If ExcludeUnlessExplicit is set in the versioning config for the document type then this will cause the normal versioning logic to kick in.  

Was thinking maybe also an Advance session command for saving with explicit versioning would be useful.