UI stuff

[Ben Collins-Sussman]

I fixed the broken jump opcode on zvm today, but now we're bumping up
against an unimplemented 'output_stream' opcode called with argument
0xFFFF. Can anyone enlighten me on how to implement this opcode?

[Atul Varma]

Looks like it's deselecting stream 1, which means that it's telling the Z-Machine not to output to the screen, I think.  This is probably where we may want to decide whether we want to use the ZOutputStreamManager class I wrote in zstreammanager.py; if we do, then we can just delegate this opcode out to it, essentially, otherwise we keep track of what streams are currently selected and so forth in ZCpu.

Want me to take a stab at implementing this one?  No worries if not.

- Atul