Save unknown tags in textual codecs

[madera]

Unknown tags should be stored, even if unprocessed, so they can be replicated if the data is ever rewritten.

For instance, Person:

<Person><UnknownTag>.......</UnknownTag></Person>

UnknownTag should be preserved for when the user (if ever) wishes to store the changed data.

[madera]

Well, even binary codecs should respect unknown blocks.