Allow a custom template to be specified for a specific form group

Use case (as breifly discussed in Zurich):

* In the CMF MenuBundle we need to be able to change the state of the form with javascript depending on the state of a checkbox (linkType).
* When the link type is changed we want to either hide or disable non-relevant fields within the group.
* As this is not something manageable with the form framework (AFAIK), we need to override the template
* Overriding the entire template involves lots of copy/paste

Example

````php
$formMapper->with('form_group.foobar', array('template' => 'CmfMenuBundle:Admin:menu_node_group.html.twig'))
->add(...)
````

This is a BC change.