Request: hookable methods to facilitate use with non-core Fieldtypes #29

It would be great if there were some hookable methods in the module that allowed for special treatment to be applied to non-core fieldtypes. I'm imagining hooks that let a developer modify the history data that is saved to the database, and modify the result of that data when it is restored to a field.

For instance, I tested with FieldtypeMapMarker and the history and diff parts seemed to work okay but the restore action failed because the saved data needs to be converted into an object of the right class.