Fix german Umlauts in Sluggable extension

I've created a version that transforms german umlauts (vocals with diaeresis, like *Ü, Ä, Ö, ü, ä* and *ö*) correctly: Don't just transform to the base vocal, but instead append an *e* to the vocal.

Since transforming the old way looks very exceptional for german people (are those diaeresis used somewhere else in the world?), this should be a useful fix.