Display default values better

For this option...

```
  option debug-opendylan? :: <boolean> = #t, // default to #f eventually
    names: #("debug-opendylan"),
    kind: <flag-option>,
    help: "Turn on debugging for Open Dylan. [%default%]";
```

the displayed help is...

```
--debug-opendylan    Turn on debugging for Open Dylan. [#t]
```

For `<boolean>` options we should output "true" or "false".