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" ] ]
```
