add norwegian stemmer (#717)

src/tokenizer/stemmer.rs

@@ -15,6 +15,7 @@ pub enum Language {
     Greek,
     Hungarian,
     Italian,
+    Norwegian,
     Portuguese,
     Romanian,
     Russian,
@@ -38,6 +39,7 @@ impl Language {
             Greek => Algorithm::Greek,
             Hungarian => Algorithm::Hungarian,
             Italian => Algorithm::Italian,
+            Norwegian => Algorithm::Norwegian,
             Portuguese => Algorithm::Portuguese,
             Romanian => Algorithm::Romanian,
             Russian => Algorithm::Russian,