Add get_zimId method to Result

get_zimId method allows the user to get the uuid of the archive from
which a result is retrieved directly from the search result itself.

include/searcher.h

@@ -49,6 +49,7 @@ class Result
   virtual int get_wordCount() = 0;
   virtual int get_size() = 0;
   virtual int get_readerIndex() = 0;
+  virtual std::string get_zimId() = 0;
 };
 
 struct SearcherInternal;