See #3401 and http://orgmode.org/manual/Macro-replacement.html
% pandoc -f org -t native
#+MACRO: HELLO /Hello, $1/
{{{HELLO(World)}}}
^D
[Para [Emph [Str "Hello,",Space,Str "World"]]]
Inverted argument order
% pandoc -f org -t native
#+MACRO: A $2,$1
{{{A(1,2)}}}
^D
[Para [Str "2,1"]]