Change to support nested cmd options based on top level commands

[farzonl]

I first made this change Tue Oct 31 17:11:24 2017 -0400. And just now realized I never tried to upstreamed this. I've found this really useful and if possible would like to upstream. This is an example of my use case from them:

class AutoCLI
{
private:
	std::unique_ptr<BaseCLIParser> CliParser;
	void initParser(int argc, char** argv)
	{
		cli::Parser parser(argc, argv);
	
		if (parser.doesArgumentExist(DecryptCLIParser::name, "--" + DecryptCLIParser::altName))
		{
			CliParser = std::unique_ptr<BaseCLIParser>(new DecryptCLIParser(argc, argv));
			return;
		}

		if (parser.doesArgumentExist(EncryptCLIParser::name, "--" + EncryptCLIParser::altName))
		{
			CliParser = std::unique_ptr<BaseCLIParser>(new EncryptedDataMarkingsCLIParser(argc, argv));
			return;
		}
	}
public:
	AutoCLI(int argc, char** argv)
	{
		initParser(argc, argv);
		
		if (CliParser)
		{
			CliParser->executeAction();
		}
	}
};

essentially I wanted a light\simple way to check the arguments lists before deciding which parser to invoke.

[FlorianRappl]

Makes sense - thanks a lot! 🍻