Add LevelGenerator.RemoveRoom()

[EliteMasterEric]

A RemoveRoom(int Column, int Row) function which removes a given room from the level generator would, in conjunction with the existing PlaceRoom() function and MC_PRE_NEW_ROOM, allow modders to create their own room layouts. The current implementation does not have any means through which a room can be prevented from generating, or replaced with a room of a different shape.

[EliteMasterEric]

You'd probably also want a setting to change the starting room index to a different room.