Set to true nullable bool arguments without the value

Consider the following class:

```csharp
class Parameters
{
[Option("bool")]
public bool Bool { get; set; }

[Option("nullable-bool")]
public bool? NullableBool { get; set; }
}
```

It works fine for such arguments: `--bool --nullable-bool true` but doesn't work for `--bool --nullable-bool`.

I suggest making it working for nullable bool types.