Add BiomeOverlay rendermode.

A "biomes" option is specified in rendermodes.py but not yet wired up!
May do unexpected things if used :-)

overviewer_core/rendermodes.py

@@ -208,6 +208,12 @@ class MineralOverlay(Overlay):
         'minerals' : ('a list of (blockid, (r, g, b)) tuples for coloring minerals', None),
     }
 
+class BiomeOverlay(Overlay):
+    name = "overlay-biomes"
+    options = {
+        'biomes' : ('a list of (biome, (r, g, b)) tuples for coloring biomes', None),
+    }
+
 class Hide(RenderPrimitive):
     name = "hide"
     options = {