Making RoomVisual an interface.

[r12f]

I found a helpful project to help us using RoomVisual in github, which is trying to extend the RoomVisual by changing its prototype: https://github.com/screepers/RoomVisual.

To make the project work for typescript, I created a typing to extend the RoomVisual: https://github.com/r12f/screeps-room-visual-extension-typings.

But since RoomVisual is declared as a class, typescript doesn't allow us to do it, so I am thinking to change this class to an interface.

I am very new in typescript, so please let me know, if it is actually a horrible idea.

[thaelina]

Hey, change the constructor parameter to optional and this would be good. See #121