exclude Matroska v5 elements from the v4 document

[robUx4]

So we can start merging v5 elements and they won't affect the v4 document.

v5 elements will likely be part of a new document that may not include elements from v1 to v4.

[robUx4]

#318 and #606 could be merged after this PR.

[robUx4]

Ping @mbunkus I'd like to merge this so I can merge #318, #606 and #659. For the Matroska specs it will be transparent. But on the (de)muxer side that means you may start adding v5 support in you case you encounter those elements.

[robUx4]

Done. We can now have v6, v7, v8, etc and the source will still produce the same v1 to v4 IETF document.