Factory for LowerCaseFilter and some language-specific variants
supported by the language parameter:
greek: GreekLowerCaseFilter
irish: IrishLowerCaseFilter
turkish: TurkishLowerCaseFilter
Real work actually done here by Sebastian on the Elasticsearch mailing list
http://elasticsearch-users.115913.n3.nabble.com/Using-the-Snowball-stemmers-tp2126106p2127111.html