Add "Best Book Move" UCI option

Is a boolean option that when set allows Stockfish
to select the best book move across the possible ones.

Feature requested by Salvo Spitaleri.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/book.h

@@ -61,7 +61,7 @@ public:
   void open(const std::string& fName);
   void close();
   const std::string file_name();
-  Move get_move(const Position& pos);
+  Move get_move(const Position& pos, bool findBestMove);
 
 private:
   Book& operator>>(uint64_t& n) { n = read_integer(8); return *this; }