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;
 };

include/xapianSearcher.h

@@ -43,6 +43,7 @@ class XapianResult : public Result
   virtual std::string get_title();
   virtual int get_score();
   virtual std::string get_snippet();
+  virtual std::string get_content();
   virtual int get_wordCount();
   virtual int get_size();