Add `--help` CLI argument

[Sohang Chopra]

Add command line help by identifying flags --help or -h. All CLI tools have these arguments, so it's a bit strange that 2048 doesn't.

[Maurits van der Schee]

True and some have -v or --version too :-)

[Ricardo Fernández Serrata]

All CLI tools have these arguments, so it's a bit strange that 2048 doesn't.

I'm unsure if that's necessary here, as the README is helpful, and the game is simple and intuitive. But it's still "good practice" in terms of design.

pedantic correction: It should be "parameters" rather than "arguments", in this case. Params are named arg values. But since help doesn't "store" any args, it should be referred-to as a "flag" or "switch"