add `allowCrossDiagnal` option

This option allows movements like this: [Imgur](http://i.imgur.com/TcawUwq.png)
This is the expected behavior in a lot of roguelike games, especially when it's hostile creatures instead of walls.

This PR only changed the AStar finder - if you think this is a good idea, I can modify other finders and their respective comments/documentations as well.