Consider adding a way to override the behavior of FieldOfView

[tgeraty]

Currently the existing behavior of FoV creates a diagonal pattern rather than a circular pattern FoV. I was going to try to override the behavior of FoV, but there's no way to do so because of how tightly bound and hidden the logic is in Map & Cell. I saw a similar issue posted on the old Bitbucket site. Also saw your post here: https://roguesharp.wordpress.com/2017/04/25/roguesharp-4-0-pre-release/ but that won't allow us to overwrite the behavior of Cell.IsInFov.