The method chill_custom_fields_is_empty take now the fields as first argument, and the customField as second argument, to be consistent with other twig methods.
Now, the CustomFieldsHelper methods and the twig function which render customFields require always a `Chill\CustomFields\Entity\CustomField`.
The possibility to add a slug and an object as argument to those method is abandoned, and was not used in other bundle (nevertheless, the method to catch a customField from his slug was buggy).
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.
