Search result objects now have a get_content method.

This was not necessary when searching in only one zim file as `url` was
enough to get the article (and so the content).

If we want to search in several zim in the same time, we need a way to get
the content directly.

include/searcher.h

@@ -46,6 +46,7 @@ class Result
   virtual std::string get_title() = 0;
   virtual int get_score() = 0;
   virtual std::string get_snippet() = 0;
+  virtual std::string get_content() = 0;
   virtual int get_wordCount() = 0;
   virtual int get_size() = 0;
 };