Add aliases option

[tommy-mitchell]

Adds flag.aliases as specified in #109:

flags: {
	unicorn: {
		type: 'string',
		shortFlag: 'u',
		aliases: ['unicorns']
	}
}
//=> Accepts `--unicorn`, `--unicorns`, or `-u`

[sindresorhus]

@tommy-mitchell Do you plan any more changes? If not, I will do a new version.

[tommy-mitchell]

I was also working on a .choices option.