Bug fix so rollup-plugin-vue works with PostCSS

Apologies if this isn't a good fix. It works for me, but it's entirely possible there is a better solution. I seem to notice in older versions of the plugin, the css files has .css extensions and now they do not. Perhaps this was the source of the issue, or I am completely wrong and the files never had .css. Hope this helps.

vuejs#241

Author: objectivehtml

src/index.ts

@@ -173,6 +173,9 @@ export default function VuePlugin(opts: VuePluginOptions = {}): Plugin {
             return require.resolve(src, { paths: [path.dirname(ref.filename)] })
           }
         }
+        else if(ref.meta.type === 'styles') {
+            return id.replace('.vue', '.vue.css');
+        }
 
         return id
       }