Add flavour attribute to book.

Fix #259
Fix kiwix/kiwix-tools#316

src/wrapper/java/book.cpp

@@ -58,6 +58,7 @@ GETTER(jstring, getPublisher)
 GETTER(jstring, getDate)
 GETTER(jstring, getUrl)
 GETTER(jstring, getName)
+GETTER(jstring, getFlavour)
 GETTER(jstring, getTags)
 GETTER(jlong, getArticleCount)
 GETTER(jlong, getMediaCount)

src/wrapper/java/org/kiwix/kiwixlib/Book.java

@@ -19,6 +19,7 @@ public class Book
   public native String getDate();
   public native String getUrl();
   public native String getName();
+  public native String getFlavour();
   public native String getTags();
   /**
    * Return the value associated to the tag tagName