data/pandoc.lua: fix Element inheritance

Extending all elements of a given type (e.g., all inline elements) was
difficult, as the table used to lookup unknown methods would be reset
every time a new element of that type was created, preventing recursive
property lookup. This is was changed in that all methods and attributes
of supertypes are now available to their subtypes.

data/pandoc.lua

@@ -33,13 +33,15 @@ local List = require 'pandoc.List'
 -- @type Element
 -- @local
 local Element = {}
+Element.__index = Element
 
 --- Create a new element subtype
 -- @local
 function Element:make_subtype(o)
   o = o or {}
   setmetatable(o, self)
-  self.__index = self
+  -- Make subtype usable as a metatable
+  o.__index = o
   return o
 end
 
@@ -57,7 +59,6 @@ function Element:new(tag, ...)
     element.c = content
   end
   setmetatable(element, self)
-  self.__index = self
   return element
 end