[fix] resolve css imports

This resolves problems with windows paths

lib/app/App.vue

@@ -8,7 +8,7 @@
 <script>
 <% if (loading) { %>import NuxtLoading from '<%= (typeof loading === "string" ? loading : "./components/nuxt-loading.vue") %>'<% } %>
 <% css.forEach(function (c) { %>
-import '<%= c.src || c %>'
+import '<%= r(c.src || c) %>'
 <% }) %>
 
 let layouts = {

lib/build.js

@@ -274,7 +274,8 @@ async function generateRoutesAndFiles () {
     const template = _.template(fileContent, {
       imports: {
         serialize,
-        hash
+        hash,
+        r
       }
     })
     const content = template(Object.assign({}, templateVars, {