Sub-stream support

[hakanai]

Sub-stream support would entail:

* The ability to mark a stream as a sub-stream, possibly processed in some way (e.g. zlib inflate).
* The ability to open that stream separately.
* The ability to annotate that stream.

The idea is that the notebook format would expand to support multiple streams with an annotation collection for each stream, plus instructions on how to unpack each stream.