Floating Island World Generation

Minor changes:

• Added lake islands, comes with sand edges and is mostly water, there is normally clay and sugar cane on the island
  
• Changed dungeon loot, replaced dirt with pumpkin/melon seeds and sugar cane
  
• Added config values to change the chance of lake islands, and the chance of clay and sugar cane on these islands
  
• Dungeons will no longer spawn on ender islands, never seen them on one because they're rare, but I imagine it would look odd

Lake islands have weird 1-block columns sometimes, will try and fix for next version