expand umlaut, eg. 'ö' -> 'oe'

[missinglink]

it's common for users speaking germanic languages to expand ö to oe, especially when stuck using an English keyboard.

an example of this is Löhningen which we can currently surface using Löhningen and the asciifolded version Lohningen but not by the term Loehningen.

other vowels are also affected, they are called "umlaut" in German, there may be an existing Lucene analyser for this which we can use.

ref: https://en.wikipedia.org/wiki/%C3%96 thanks @ralphiech for reporting

[ralphiech]

This essentially is relevant for the following chars

ü ö ä Ä Ü Ö

[orangejulius]

Like https://github.com/pelias/schema/issues/65#issuecomment-237240507, this is now possible thanks to our alt-name support. It's something we should look at again.