PIM-2300 : add a dedicated transformer for atribute option, allows to
throw an exception if the attribute is not known
PIM-2300 : declare the new transformer as tagged service
PIM-2300 : fix CS
PIM-2300 : update changelog
PIM-2300 : add spec for attribute option transformer to prepare the
future
Merge branch 'master' into PIM-2300
Merge pull request #999 from akeneo/PIM-2300

PIM-2300 : add an atribute option tranformer