Nurikabe: Extend Black Between Regions

[Bram28]

Extend this rule so that the user is allowed to place a black between two white regions when:

1. One region contains a number
2. The other region does not
3. Placing a white between the regions would create one region with a size greater than the number

[mmellone]

Fixed a bug with this. Appears to work well now.
e4a8eec

[Bram28]

Great, thanks!