title: vee-validate Validating a non-existing field
body: <p>When validating a group of fields, an error is shown for fields that are hidden in the DOM with a v-if attribute.</p>
  
  <p>For example, I've got 3 fields:</p>
  
  <pre><code>&lt;input name="foo" v-validate="'required'" /&gt;
  &lt;input name="bar" v-validate="'required'" /&gt;
  &lt;input v-if="showMe" name="foobar" v-validate="'required'" /&gt;
  </code></pre>
  
  <p>When I run my submit func I'm checking that none of the fields contain errors:</p>
  
  <pre><code>this.$validator.validateAll().then(() =&gt; {
    ... my check
  });
  </code></pre>
  
  <p>I see the following error:</p>
  
  <pre><code>Uncaught (in promise) Error: [vee-validate] Validating a non-existent field: "#3". Use "attach()" first.
  </code></pre>
  
  <p>Detaching and reattaching my 'foobar' field as I update the "showMe" state seems verbose. Especially for my larger multistep forms.</p>
  
  <p>Is there an easier way to prevent this error from appearing?</p>