Consider custom parsers/converters/handler for types?

I would love to be able to do:

```c#
public class Options
{
[Option('o', "out", Required = true, HelpText = "Output file")]
public string OutFile { get; set; }
[Option('d', "duration", Required = false, Default = "1:00:00", HelpText = "Duration")]
public TimeSpan Duration { get; set; }
[Option('e', "endpoint", Required = true, HelpText = "IP Endpoint (:)")]
public IPEndPoint Endpoint { get; set; }
}
```

Where I can use types like `TimeSpan` and `IPEndPoint` or any other type. Ofcourse, we would need to be able to specify a parser/converter/handler/whatever that takes the value and converts it to the desired type. I imagine it be like:

```c#
public class Options
{
[Option('o', "out", Required = true, HelpText = "Output file")]
public string OutFile { get; set; }
[Option('d', "duration", Required = false, Default = "1:00:00", HelpText = "Duration", Converter = typeof(MyTimeSpanConverter))]
public TimeSpan Duration { get; set; }
[Option('e', "endpoint", Required = true, HelpText = "IP Endpoint (:)", Converter = typeof(MyIPEndpointConverter))]
public IPEndPoint Endpoint { get; set; }
}
```

Next these 'converters' should implement something like an IConvert that defines a method `T Convert(string value)` where they take a string (the commandline argument) and convert to the desired type. Another option would be to not use the attributes but be able to specify handlers like:

```c#
Parser.Default
.Register()
.Register()
.ParseArguments(args)
....
```

Would that be something that you would consider? (Maybe not exactly following the idea given above, but the broader idea of being able to handle custom types).

実装ファイルやテストは指定されていません。Parser.Default と ParseArguments から始め、現在オプション文字列がどのように変換されているかを追跡し、提案されている属性ベースの Converter と登録のアプローチを比較してください。TimeSpan や IPEndPoint などのカスタム型を、確定してテスト済みの API を通じて処理できるようになれば完了です。

索引モデルが issue の本文から書いたものです。