Support + (plus) sign for flags to UNSET a flag often used by Linux

>All of the single letter options that have a corresponding name can be used as an argument to the -o option. The set -o name is provided next to the single letter option in the description below. Specifying a dash “-” turns the option on, while using a plus “+” disables the option.

Meaning, assume you have the following option:

[Option('x', "XXXX", Required = false, Default = true)]
public bool X { get; set; }

And you would pass `+x`, then the result should be `False`:

app.exe +x

This seems like a better approach then a nullable bool:

[Option('x', "XXXX", Required = false, Default = true)]
public bool? X { get; set; }

As that requires to pass:

app.exe -x false

在命令列選項解析入口中尋找現有的單字母 flag 處理，以及涵蓋選項前綴的測試。驗證宣告為預設值 true 的選項在以 +x 呼叫時會變為 false，同時保留文件所述的 -x 行為；為所要求的語法新增涵蓋測試。

由索引模型根據 Issue 內容生成。