made form extensions extensible (closes #398)

doc/providers/form.rst

@@ -143,6 +143,14 @@ form by adding constraints on the fields::
         ))
         ->getForm();
 
+You can register form extensions by extending ``form.extensions``::
+
+    $app['form.extensions'] = $app->share($app->extend('form.extensions', function ($extensions) use ($app) {
+        $extensions[] = new YourTopFormExtension();
+
+        return $extensions;
+    }));
+
 Traits
 ------
 

src/Silex/Provider/FormServiceProvider.php

@@ -46,7 +46,7 @@ class FormServiceProvider implements ServiceProviderInterface
 
         $app['form.secret'] = md5(__DIR__);
 
-        $app['form.factory'] = $app->share(function () use ($app) {
+        $app['form.extensions'] = $app->share(function () use ($app) {
             $extensions = array(
                 new CoreExtension(),
                 new CsrfExtension($app['form.csrf_provider']),
@@ -61,7 +61,11 @@ class FormServiceProvider implements ServiceProviderInterface
                 }
             }
 
-            return new FormFactory($extensions);
+            return $extensions;
+        });
+
+        $app['form.factory'] = $app->share(function () use ($app) {
+            return new FormFactory($app['form.extensions']);
         });
 
         $app['form.csrf_provider'] = $app->share(function () use ($app) {