[EAD-8] Add support for custom content preprocessors and postprocessors

[smiakchilo]

As an AnyDiff user, I may sometimes have the need to preprocess content that goes into comparison in a special way. E.g., to remove a certain node from XML file, merge lines, etc.
Changes made by such a preprocessor must be available for revering with use of a corresponding postprocessor that prepares content for displaying after comparison markers are set.

This feature should be available from the API.

[smiakchilo]

Implemented in v.1.0.0