Add possibility to hide empty value in customfield group view rendering

A new parameter is defined :

```
chill_custom_fields:
    show_empty_values_in_views = true|false
```

A new method is added to CustomFieldInterface: `isEmptyValue`. To ease
the dev of new classes, an AbstractCustomField class is created, which
implements the most commons function (currently, only isEmptyValue).

A new Twig Filter is added: `chill_custom_field_is_empty`

The twig filter `chill_custom_fields_group_widget` has a new possibility
in array option : `show_empty`. Default to
chill_custom_fields.show_empty_values_in_view. May be forced by
true/false.

CustomFields/CustomFieldInterface.php

@@ -58,4 +58,12 @@ interface CustomFieldInterface
      * @return \Symfony\Component\Form\FormTypeInterface|null the form type
      */
     public function buildOptionsForm(FormBuilderInterface $builder);
+    
+    /**
+     * Return if the value can be considered as empty
+     * 
+     * @param mixed $value the value passed throug the deserialize function
+     * @param CustomField $customField
+     */
+    public function isEmptyValue($value, CustomField $customField);
 }