Add autogenerate-to-visibilitylimits option to allow automatic priming/generating of chunks within defined visibility area

Avoid rendering tiles with nothing but empty chunks
Limit size of hash-file cache, and move hash files to world-specific tile directories

src/main/java/org/dynmap/DynmapWorld.java

@@ -27,6 +27,7 @@ public class DynmapWorld {
     public ConfigurationNode configuration;
     public List<Location> seedloc;
     public List<MapChunkCache.VisibilityLimit> visibility_limits;
+    public boolean do_autogenerate;
     public MapChunkCache.HiddenChunkStyle hiddenchunkstyle;
     public int servertime;
     public boolean sendposition;