Allow unbounded generations and fitness

[aprescott]

It would be good to be able to just let the thing run without bound, possibly allowing snapshots to be saved to see full-sized rendered images.

[phl]

Yes, although you do get into diminishing returns pretty quickly - I've let one run for something like 300,000 gens and the cv confidence never got above 35. And it still didn't look any better.

The only cut-off value that's hardcoded at the moment is to bail if there's no improvement for 1000 generations, guess I could make that configurable option though.