add missing todomvc-common CSS

Author: sindresorhus

index.html

@@ -4,6 +4,7 @@
 		<meta charset="utf-8">
 		<meta name="viewport" content="width=device-width, initial-scale=1">
 		<title>Template • TodoMVC</title>
+		<link rel="stylesheet" href="node_modules/todomvc-common/base.css">
 		<link rel="stylesheet" href="node_modules/todomvc-app-css/index.css">
 		<!-- CSS overrides - remove if you don't need it -->
 		<link rel="stylesheet" href="css/app.css">