Keep saved state in browser local storage. Ideally also have a way to save image states in the cloud too; maybe v2.0.

May mitigate the "ctrl-W" problem further (#47).

Not sure how to get it working in the presence of having a "save state" and then clicking a new jsbeeb URL: should that overwrite the last save? Or hash by URL? In my mind I'd like to be able to go back to jsbeeb and have it carry on from wherever it was last, but that might not work with bookmarked URLs.

Needs some thought, ideas welcomed!