Display help without requiring ParseArguments

**Issue by [Steve887](https://github.com/Steve887)**
_Wednesday Jan 20, 2016 at 01:30 GMT_
_Originally opened as https://github.com/gsscoder/commandline/issues/281_

----

I'm trying to print the help text for my options without having to force a parser error.

Currently, it seems the only way to print the help text is to call something like

```
Parser.Default.ParseArguments(new string[] { "verb", "--help" });
```

I see there's `HelpText.AutoBuild` but that seems to require a NotParsed result.

Is there a way to write the help text for an options class without having to parse arguments? Maybe something like `HelpText.DisplayHelp`?

首先阅读 Parser.Default.ParseArguments 和 HelpText.AutoBuild，并以 Options 类和现有的 --help 示例作为切入点。确定如何在不强制产生 NotParsed 结果的情况下生成帮助文本，然后验证生成的输出是否与通过解析生成的帮助内容一致。

由索引模型根据 Issue 内容生成。