Keep track of cash

First attempt at keeping track of where cash goes using the new hidden
'-cash' account.

plugins/withdraw

@@ -18,8 +18,9 @@ sub amount($self, $cart, $arg, @) {
     my $amount = parse_amount($arg);
     defined $amount or return REJECT, "Invalid amount";
 
-    $cart->add(-$amount, "Withdrawal",
-        { is_withdrawal => 1 });
+    $cart
+        ->add(-$amount, "Withdrawal", { is_withdrawal => 1 })
+        ->add_contra("-cash", +$amount, "Withdrawn by \$you");
 
     return ACCEPT;
 }