Add custom swamp biomes

[jasonw4331]

I may be incorrect in saying this, but I'm pretty sure that swamp biomes have trees and lily pads. This is just a lake with the incorrect biome...

[AvgZing]

WOAH

[AvgZing]

I actually consider that cool, but that may just be me...

[Ad5001]

I'm using the defaut pocketmine swamp. That's the reason.