Handle \write18{..} as raw tex:
```
% pandoc -t native
\write18{git --version}
^D
[ RawBlock ( Format "tex" ) "\\write18{git --version}" ]
```

```
% pandoc -f latex+raw_tex -t native
\write18{git --version}
^D
[ RawBlock ( Format "latex" ) "\\write18{git --version}" ]
```