Basic mention support.

I still have to think about how to integrate them in the state system...

src/components/nav_panel/nav_panel.vue

@@ -2,11 +2,16 @@
   <div class="nav-panel">
     <div class="panel panel-default">
       <ul>
-        <li v-if='loggedIn'>
+        <li v-if='currentUser'>
           <router-link to='/main/friends'>
             Timeline
           </router-link>
         </li>
+        <li v-if='currentUser'>
+          <router-link :to="{ name: 'mentions', params: { username: currentUser.screen_name } }">
+            Mentions
+          </router-link>
+        </li>
         <li>
           <router-link to='/main/public'>
             Public Timeline