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

Search the command-line option parsing entry point for existing single-letter flag handling and tests covering option prefixes. Verify that an option declared with a true default becomes false when invoked as +x, while preserving the documented -x behavior; add coverage for the requested syntax.

Written by the indexing model from the issue text.