title: Yaml numeric keys: can't start with zero?
body: <p>Let's say I have this <code>.yml</code> file used for Symfony2 i18n</p>
  
  <pre><code>month:
    01: Jan
    02: Feb
    03: Mar
    04: Apr
    05: May
    06: Jun
    07: Jul
    08: Aug
    09: Sep
    10: Oct
    11: Nov
    12: Dec
  </code></pre>
  
  <p>In twig, for tanslation, I will use something like</p>
  
  <pre><code>{% set month = my_date|date("m") %}
  {{ ('month.'month)|trans }}
  </code></pre>
  
  <p>In this scenario, all months starting with "0" will not be translated. I've read yaml documentation (not so deep, but I've searched on the internet) and I haven't find any page saying that a key couldn't start with zero (and it seems not to be possible for a key start with zero). </p>
  
  <h2>So, my questions are:</h2>
  
  <ul>
  <li>Is that a yaml issue?</li>
  <li>If this isn't an issue, why keys could not start with "0"?</li>
  </ul>