Add background attribute to default sources (#408)

Author: s0ph1e

lib/config/defaults.js

@@ -32,7 +32,8 @@ const config = {
 		{ selector: 'audio source', attr: 'src' },
 		{ selector: 'audio track', attr: 'src' },
 		{ selector: 'frame', attr: 'src' },
-		{ selector: 'iframe', attr: 'src' }
+		{ selector: 'iframe', attr: 'src' },
+		{ selector: '[background]', attr: 'background' }
 	],
 	subdirectories: [
 		{ directory: 'images', extensions: ['.png', '.jpg', '.jpeg', '.gif', '.webp'] },