title: vue js disable input if value hasn't changed
body: <p>I have an input with an initial value:</p>
  
  <pre><code>&lt;input type="text" v-model="name" ref="input" /&gt;
  &lt;button type="submit" :disabled="$refs.input.defaultValue == $refs.input.value"&gt;Submit&lt;/button&gt;
  </code></pre>
  
  <p>However the <code>disabled</code> binding gives an error: <code>Cannot read property defaultValue of undefined</code>.
  Best way to do this without spamming vm.data too much?</p>