Allow encoding of an activity with configuration preset durations

fix #11

The activity form allow to receive pre-configured activity durations.

Those duration can be configured through the option
chill_activity.form.time_duration :

```yaml

chill_activity:
   form:
      time_duration:
         - { label: '12 minutes', seconds: 720 }
         # - ...

```

If a pre-existing activity receives a different time, the time is added
to the list of pre-configured duration time.

DependencyInjection/ChillActivityExtension.php

@@ -42,6 +42,8 @@ class ChillActivityExtension extends Extension implements PrependExtensionInterf
     {
         $configuration = new Configuration();
         $config = $this->processConfiguration($configuration, $configs);
+        
+        $container->setParameter('chill_activity.form.time_duration', $config['form']['time_duration']);
 
         $loader = new Loader\YamlFileLoader($container, new FileLocator(__DIR__.'/../Resources/config'));
         $loader->load('services.yml');