Umlaute with stemmer de

**Describe the bug**
"grüne" will be "grun" with the german stemmer. But it must be "grün"

**To Reproduce**

```
const natural = require('natural');
const stemmer = natural.PorterStemmerDe;
console.log(stemmer.stem("grüne"));
```

Output: grun

**Expected behavior**

Output grün
