title: Add _format automatically to all routes yml Symfony2
body: <p>I am building a REST API with Symfony2 and JMSSerializerBundle.</p>
  
  <p>At the beginning, it was JSON-only formatted because I didn't have the time to implement XML for instance,</p>
  
  <p>But now that I have this time, I would like to implement it.</p>
  
  <p>I wonder how to manage the _format parameter,
  Should I manually update all the routes, adding this parameter with JSON as default, or is there any way to update it manually ?</p>
  
  <p>Basically, I have almost tens of routes like this :</p>
  
  <pre><code>my_rest_bundle_api_users:
      pattern:  /
      defaults:
          _controller: "MyRestBundle:V1/Entities/Users:list"
      requirements:
          _method: get
  </code></pre>
  
  <p>I just would not have to edit all these routes manually, if possible !</p>
  
  <pre><code>my_rest_bundle_api_users:
      pattern:  /.{_format}
      defaults:
          _controller: "MyRestBundle:V1/Entities/Users:list"
          _format: json
      requirements:
          _method: get
          _format: json|xml
  </code></pre>