message: elle.Defaulted: Pretty print.
diff: @@ -74,6 +74,15 @@ namespace elle
  bool _set = false;
  };
  + template <typename T>
  + std::ostream&
  + operator <<(std::ostream& out, Defaulted<T> const& t)
  + {
  + elle::fprintf(out, "%s", t.get());
  + if (!t)
  + elle::fprintf(out, " (default)");
  + return out;
  + }
  template <typename T>
  Defaulted<T>
  