Options aren't found if <argument> is too long

Consider the usage text:

```
Usage:
  foo <argument>
  foo --help

Options:
  -h, --help        Print this message.
  -v, --verbose     Print extra warnings and information.
```

When invoked as `foo --help` or as `foo -h`, `(isPresent args (longOption "help"))` will return false. This is unsettling.
