Rename `alias` option to `shortFlag`

I never liked the alias wording, which was inherited from minimist. I think it would be better to have a more explicit and semantic name called shortFlag.

We could then also add a aliases option where users can specify actual aliases and not just limited to one:

{
	flags: {
		unicorn: {
			type: 'string',
			default: 'love',
			shortFlag: 'u',
			aliases: [
				'--rainbow',
				'-r'
			]
		}
	}
}

Could use some feedback on this proposal.

[sindresorhus]

@LitoMore It’s not. shortFlag is there to semantically indicate what’s the short version of the flag. aliases is just any kind of alias long/short/medium/thin/thick/unicorny. This distinction will be especially important when we start generating the help output.

[sindresorhus]

@BendingBender That would be a big breaking change and IMHO not for the better. I also want to make a clear distinction between an alias and a short-flag.