Doctool may swallow errors due to unhandled async functions

Basically, this code is not good:

	fs.readFile(filename, 'utf8', async (er, input) => {
	if (er) throw er;

	const content = unified()
	.use(markdown)
	.use(html.preprocessText)
	.use(json.jsonAPI, { filename })
	.use(html.firstHeader)
	.use(html.preprocessElements, { filename })
	.use(html.buildToc, { filename, apilinks })
	.use(remark2rehype, { allowDangerousHTML: true })
	.use(raw)
	.use(htmlStringify)
	.processSync(input);

	const basename = path.basename(filename, '.md');

	const myHtml = await html.toHTML({ input, content, filename, nodeVersion });
	const htmlTarget = path.join(outputDir, `${basename}.html`);
	fs.writeFileSync(htmlTarget, myHtml);

	const jsonTarget = path.join(outputDir, `${basename}.json`);
	fs.writeFileSync(jsonTarget, JSON.stringify(content.json, null, 2));
	});

Issues with that:

Async function set as a callback (unhandled)
sync I/O calls inside an async function
unified is synchronously invoked still

This is all the direct result of f4f856b, which is the result of something else. In hindsight, I wish I payed a bit more attention in my review of that.

Unfortunately my focus is elsewhere, so someone else should probably try to clean this up.