Add method to get value of tag from a book.

Only the `getTagStr` method is available on android because we need a
proper exception handling on wrapping side.

Fix #298

src/wrapper/java/book.cpp

@@ -66,4 +66,11 @@ GETTER(jstring, getFavicon)
 GETTER(jstring, getFaviconUrl)
 GETTER(jstring, getFaviconMimeType)
 
+METHOD(jstring, Book, getTagStr, jstring tagName) try {
+  auto cRet = Book->getTagStr(jni2c(tagName, env));
+  return c2jni(cRet, env);
+} catch(...) {
+  return c2jni<std::string>("", env);
+}
+
 #undef GETTER