--static now also copies background images (#461)

lib/static.js

@@ -20,6 +20,7 @@ const featuredSlide = require('./featured-slide');
 const files = new Set();
 
 const htmlImageRE = /<img.+src=["'](.+?)["'](?:.+?)>/g;
+const htmlImageBackgroundRE = /data-background-image=["'](.+?)["']/g;
 
 const relativeDir = (from, to) => path.relative(from, to).replace(/^\.\./, '.');
 
@@ -71,9 +72,12 @@ const copyAssetsAndWriteFile = async (sourceDir, file, targetDir) => {
   const base = relativeDir(file, '.');
   const markup = await renderFile(path.join(sourceDir, file), { base });
 
-  let image;
   const html = md.marked(markdown.toString());
-  while ((image = htmlImageRE.exec(html))) {
+  const images = html.matchAll(htmlImageRE);
+  const backgroundImages = html.matchAll(htmlImageBackgroundRE);
+  const allImages = [...images, ...backgroundImages];
+
+  for (let image of allImages) {
     const [, imgPath] = image;
     if (!isAbsoluteURL(imgPath)) {
       const relPath = path.join(path.dirname(file), imgPath);