feat(plugin): support id as a function (asyncID) (#43)

Author: ricardogobbosouza

lib/plugin.js

@@ -1,11 +1,15 @@
 import Vue from 'vue'
 import VueAnalytics from 'vue-analytics'
 
-export default (ctx, inject) => {
+export default async (ctx, inject) => {
   const options = <%= serialize(options) %>
 
+  if (typeof options.asyncID === 'function') {
+    options.id = await options.asyncID(ctx)
+  }
+
   Vue.use(VueAnalytics, {...{ router: ctx.app.router }, ...options})
-  
+
   ctx.$ga = Vue.$ga
   inject('ga', Vue.$ga)
 }