Support --help switch

[nfischer]

We have usage info, but I don't think this is wired up to a --help switch. This would be helpful when using the tool through an npm script (ex. npm run release:patch -- --help), which wouldn't otherwise permit you to pass an invalid argument.

[nfischer]

Looks like this was already fixed:

release/index.js

Lines 71 to 74 in d68d05f

	if (argv.help) {
	usage();
	exit(0);
	}