Add modular world generation

[bsergent]

Once #27 is completed, there will need to be a world generation system so that the sandbox is not completely empty. As this is a library, the generators will need to be very abstract and modular. I'm thinking something like decorators that occur after the main generation that only modify the properties of existing tiles, such as adding puddles and vines to dungeon corridors or adding loot to chests. Also, there will need to be some sort of base structure, something that will actually carve out dungeons, caves, etc. And then something in between for things like trees, ores, etc. With the structure generation, it would be nice if an abstract tree could be created that denoted how the structure should play, but not how it should physically be placed. Like, how many rooms there should be before finding a key, back-tracking, going through a locked gate, and eventually reaching the end while the dynamics of the level change without the level physically changing. This blueprint should actually affect the structural and population steps.