defaults deep options

This will help modules easily append to options without need to checking and creating arrays.

lib/nuxt.js

@@ -18,7 +18,12 @@ class Nuxt {
     var defaults = {
       dev: true,
       env: {},
-      head: {},
+      head: {
+        meta: [],
+        link: [],
+        style: [],
+        script: []
+      },
       plugins: [],
       css: [],
       modules: [],
@@ -50,7 +55,11 @@ class Nuxt {
         webpack: {},
         chokidar: {}
       },
-      build: {}
+      build: {
+        postcss: [],
+        vendor: [],
+        plugins: []
+      }
     }
     // Sanitization
     if (options.loading === true) delete options.loading