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/CustomFieldLongChoice.php

@@ -33,7 +33,7 @@ use Symfony\Bridge\Twig\TwigEngine;
  *
  * @author Julien Fastré <julien.fastre@champs-libres.coop>
  */
-class CustomFieldLongChoice implements CustomFieldInterface
+class CustomFieldLongChoice extends AbstractCustomField
 {
     /**
      *