Previously, ContentModifier and ContentModifyingOperationPreprocessor
were package-private. This meant that anyone wishing to perform content
modification during preprocessing had to reimplement much of the
existing content modifying preprocessor, that than just having to
implement ContentModifier.
This commits make both ContentModifier and
ContentModifyingOperationPreprocessor public so that all custom
content modification is now a matter of implementing a custom
ContentModifier and creating an instance of
ContentModifyingOperationPreprocessor.
Closes gh-121
ebab967e43