Porters stemming algorithm for Denmark

 The stem function takes a scalar as a parameter and stems the word
 according to Martin Porters Danish stemming algorithm,
 which can be found at the Snowball website: http://snowball.tartarus.org/.
 .
 Lingua::Stem::Snowball::Da also supports caching if you pass the use_cache
 option when constructing a new L:S:S:D object.