cmdopt2: add tribool support

cmdopt2/args.hpp

@@ -9,6 +9,7 @@
 #pragma once
 
 #include <cruft/util/cast.hpp>
+#include <cruft/util/bool.hpp>
 #include <cruft/util/concepts/string.hpp>
 #include <cruft/util/debug/assert.hpp>
 #include <cruft/util/parse/value.hpp>
@@ -30,7 +31,7 @@ namespace cruft::cmdopt2 {
 
 
     template <typename T>
-    concept parseable = std::is_arithmetic_v<T>;
+    concept parseable = std::is_arithmetic_v<T> or std::is_same_v<T, cruft::tribool>;
 
 
     template <typename BaseT>