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

在命令行选项解析入口中查找现有的单字母 flag 处理，以及涵盖选项前缀的测试。验证声明为默认值 true 的选项在以 +x 调用时会变为 false，同时保留文档所述的 -x 行为；为所请求的语法添加覆盖测试。

由索引模型根据 Issue 内容生成。