Suggestion: implement the xmega and new attiny parts

[wjl]

You might consider implementing the xmega and newer tinyavr architectures so that those parts could be simulated. I use both of these parts quite a bit and it would be awesome to see them supported in simavr.

The newer architectures only add a few instructions, which would probably be pretty easy to add. There are also some peripheral changes, but those could always be added later.