Support for implicit empty string

I want to design an api with a bool-like option which can optionally contains a name. Below are my commands I wanted to work with:

``` shell
// 1. Call MyFeature without adding a project
myfeature

// 2. Call MyFeature and add a project with default naming
myfeature --add-project

// 3. Call MyFeature and add a project with a specific name
myfeature --add-project "myprojectname"
```

Below is my options class:

``` csharp
[Verb("myfeature")]
public class MyOptions
{
[Option("add-project", Required = false)]
public string AddProjectName { get; set; }

public bool AddProject => AddProjectName != null;
}
```

My problem is that the 2. solution is not working, because the engine forces the user to set a string arg. In order to set an empty string he has to be give an empty "". But that would not be an intuitive api.

Is there already a solution for this design problem? Can I disable the string-check for this item? I already set "Required" to false.

该 issue 没有指定源文件、测试或入口点。首先跟踪字符串值选项的解析方式，以及缺少值时的拒绝方式。当 --add-project 可以在不带值的情况下工作，同时 --add-project "myprojectname" 仍保留所提供的名称时，即可视为完成。

由索引模型根据 Issue 内容生成。