Add support for centralized state management

- Install vuex
  ``` bash
  npm add vuex
  ```

- Create `store.js` file in `src/` directory
  Register `vuex` plugin
  Expose a `Store` instance

- Use store in the application
  Add `store` to Vue constructor options

package.json

@@ -7,7 +7,8 @@
   "dependencies": {
     "marked": "^0.3.12",
     "vue": "^2.5.13",
-    "vue-router": "^3.0.1"
+    "vue-router": "^3.0.1",
+    "vuex": "^3.0.1"
   },
   "devDependencies": {
     "@vue/cli": "^3.0.0-alpha.5",

src/index.js

@@ -1,9 +1,11 @@
 import Vue from 'vue'
 import App from './App.vue'
 import router from './router'
+import store from './store'
 
 new Vue({
   el: '#app',
   router,
+  store,
   render: h => h(App)
 })

src/store.js

@@ -0,0 +1,16 @@
+import Vue from 'vue'
+import Vuex, { Store } from 'vuex'
+
+Vue.use(Vuex)
+
+const getters = {}
+const state = () => ({})
+const mutations = {}
+const actions = {}
+
+export default new Store({
+  getters,
+  state,
+  mutations,
+  actions
+})