Implement extra_question_tables functionality like extra_question_fields

Tim, preg and other question under my development require extra tables, which will store question-specific data.

I want to implement extra_question_tables functionality to automatically get/save options, delete question, backup/restore and XML import/export for such tables.

I could either test it on my questions, or upgrade numerical to use it for units handling.

Please let me know, what you think about it and will you check in such patch if I'll write it.