title: Vue.js: How to concatenate v-model value
body: <p>How to make this work:</p>
  
  <pre><code>&lt;div v-for="index in 4"&gt;
    &lt;input v-model="'invoice_' + index" &gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>just like this:</p>
  
  <pre><code>&lt;div v-for="index in 4"&gt;
    &lt;input v-model="invoice_1" &gt;
  &lt;/div&gt;
  </code></pre>