title: Allow the addition of new values to a ChoiceType Field
body: <p>I use <code>Form Component</code> and have a <a href="http://symfony.com/doc/current/reference/forms/types/choice.html" rel="nofollow noreferrer">ChoiceType field</a> on the form which is rendered to a select field.
  On the client-side I use <a href="http://select2.github.io/" rel="nofollow noreferrer">select2 plugin</a> which initializes a select with <code>tags: true</code> allowing the addition of new values to it.<br />
  But if I add a new value then a validation on the server fails with an error</p>
  <blockquote>
  <p>This value is not valid.</p>
  </blockquote>
  <p>because the new value isn't in the choice list.</p>
  <p>Is there a way to allow the addition of new values to a <code>ChoiceType field</code>?</p>