Leading tabs must be preserved in org mode *src* blocks with the `-i`
flag when pandoc is called with `-p`/`--preserve-tabs`.

```
% pandoc -f org -t native --preserve-tabs
#+begin_src makefile -i
%.o: %.cpp
	$(CXX)	-o	$@	$<
#+end_src
^D
[ CodeBlock
    ( "" , [ "makefile" ] , [] )
    "%.o: %.cpp\n\t$(CXX)\t-o\t$@\t$<\n"
]
```
