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.

Resources/config/services.yml

@@ -106,6 +106,8 @@ services:
         class: Chill\CustomFieldsBundle\Templating\Twig\CustomFieldsGroupRenderingTwig
         calls:
             - [setContainer, ["@service_container"]]
+        arguments:
+            - "%chill_custom_fields.show_empty_values%"
         tags:
             - { name: twig.extension }