set clue modifier to 0 by default

include/command_line_interface.h

@@ -20,7 +20,7 @@ namespace Hanabi {
     boost::optional<bool> interactive {};
     bool quiet { false };
     // If this holds std::monostate, then all clue numbers are evaluated
-    std::variant<std::monostate, clue_t> clue_spec;
+    std::variant<std::monostate, clue_t> clue_spec {static_cast<clue_t>(0)};
     bool recursive { false };
   };