Default: Destroy flammable items when in fire or lava

The check for igniters (fire/lava) will be performed every 10 secs
if the item is flammable.

if the item is found to be in lava it will then burn up and
disappear in a smoke puff.

If a non-lava igniter node is found, a combination of `flammable`
value of the item and `igniter` group value of the node will be used
to determine the chance for the item to be removed.

mods/default/README.txt

@@ -183,6 +183,10 @@ Pithydon (CC BY-SA 3.0)
   default_coral_orange.png
   default_coral_skeleton.png
 
+Ferk (CC0 1.0)
+  default_item_smoke.png
+  default_item_smoke.ogg, based on sound by http://opengameart.org/users/bart
+
 Glass breaking sounds (CC BY 3.0):
   1: http://www.freesound.org/people/cmusounddesign/sounds/71947/
   2: http://www.freesound.org/people/Tomlija/sounds/97669/