add `Uri` to `ArgumentConverter.StringConverters`

Right now, if you have:
```cs
public class UrlOption : Option<Uri>
{
  public UrlOption() : base("--url")
  {
    Required = true;
  }
}
```

an `InvalidOperationException` is thrown, with the message:
```
Cannot parse argument 'http://example.com' for option '--url' as expected type 'System.Uri'.
```