Can't get first slide as feature image

[ooker777]

If in your option there is no feature image, then this line

reveal-md/lib/featured-slide.js

Lines 25 to 30 in 0f1c75a

	export default async (initialUrl, targetDir) => {
	const { featuredSlide } = getOptions();
	if (!featuredSlide || !puppeteer) {
	return;
	}

will return and you don't have any feature image.

If --featured-slide 0 is set, then this function

reveal-md/lib/featured-slide.js

Lines 18 to 21 in 0f1c75a

	const getSlideAnchor = featuredSlide => {
	const [slide, subslide] = featuredSlide.split('-').map(slide => _.parseInt(slide, 10));
	return `${isNaN(slide) ? '' : '#/' + slide + (isNaN(subslide) ? '' : '/' + subslide)}`;
	};

will make the URL to be:

${initialUrl}#/0

and this will return error. I try to change the condition to:

isNaN(slide) || slide === 0 

and the URL is just:

${initialUrl}

which should work, but it still get error. I have no idea on how to fix this.