Multiple long arguments

Oftentimes it is useful to offer multiple long arguments for an option. A common case is a lengthy long argument for which you want to have a shorter version but the 1-char is not possible (e.g. because it's already assigned to other option).

For example, I have an option `DisableResourceWrites` with a long arg `--disableResourceWrites`, for which I would like it to shorten to `--drw` (`-d`, `-r` and `-w` are already taken).

I would like to use it like:
```cs
[Option(new [] { "disableResourceWrite", "drw" }, Required = false, HelpText = "Download resources even if they are already downloaded.")]
```

The help would show up like:

```
--disableResourceWrite, --drw Download resources even if they are already downloaded.
```

The unparsing would just choose the first long arg when short names are not preferred.

首先跟踪选项声明的表示方式、在帮助中呈现的方式，以及在 unparsing 期间的处理方式。添加对多个长名称的支持，使两个名称都出现在帮助中；当不优先使用短名称时使用第一个名称，并确保现有的单名称选项继续正常工作。

由索引模型根据 Issue 内容生成。