Extended help information

I think that it would be nice if we could display additional text about application if someone types `MyApp --help`, and additional help for verb if someone types `MyApp Verb --help`.

I know there is `HelpText` property on `BaseAttribute`. It is nice for communicating what given option does, however I think that it is not suitable for extended, in-depth explanation. I would see it like that:

```
MyApp.exe help
MyApp 1.0.0
Copyright (C) 2018 MyApp

verb1 Help text for verb 1
verb2 Help text for verb 2
help Display more information on a specific command.
version Display version information.

Long explanation about app
```

```
MyApp.exe help verb1
MyApp 1.0.0
Copyright (C) 2018 MyApp

verb1 Help text for verb 1
verb2 Help text for verb 2
help Display more information on a specific command.
version Display version information.

Long explanation about verb1
```

首先阅读 BaseAttribute 和现有的帮助输出行为。确定对于两个示例命令，应用程序级和动词级的扩展文本应如何表示和显示，然后验证两种帮助形式都能生成所请求的详细说明。

由索引模型根据 Issue 内容生成。