Negative numbers with siunitx

```
% pandoc -f latex -t native
\SI{123}{\celsius}

^D
[Para [Str "123\160\176C"]]
```

```
% pandoc -f latex -t native
\SI{-123}{\celsius}

^D
[Para [Str "\8722\&123\160\176C"]]
```

```
% pandoc -f latex -t native
\SI{+123}{\celsius}

^D
[Para [Str "123\160\176C"]]
```


