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.

Start by tracing how option declarations are represented, rendered in help, and handled during unparsing. Add support for multiple long names so both names appear in help, the first is used when short names are not preferred, and existing single-name options continue to work.

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