Import/export for products (CSV)

Feature/import export product in csv (#196)

* add import/export of products

* use context in behat

* fix returns type

* enable products import/export behat tests

use (new) comma separator

add cli behat tests

update README.md

fix return type and typo

fix exportKeysAvailable

clean not need store data

importer add transformer

improve product import/export

update for new controller style

allow to remove product attribute value

update for sylius 1.4

* add images channels locals

add images channels locals

* upgrade feature for tests imports

fix phpstan version

* set length to product datas

* add variant price

* upgrade to clean code

* update channel price

* fix export images

* fix export images

* fix export images

* fix export price

* use ImportResultLoggerInterface

* fix ImporterResultInterface