title: Vue property not valid with underscore
body: <p>I was doing some tests and I noticed that my properties are not valid when I used underscore.</p>
  
  <p>Example:</p>
  
  <pre><code>new Vue({
  el : "#form",
  
  data: {
      errors: [],
      _username: '',
      _password: ''
  });
  </code></pre>
  
  <p>on html file:</p>
  
  <pre><code>&lt;input class="uk-input" type="text" v-model="_username" &gt;
  &lt;input class="uk-input" type="password" v-model="_password"&gt;
  </code></pre>
  
  <p>With the code above the app won't render. If I remove the underscore it will work, does someone knows why this happens?</p>