Simplify adding other attribute value handlers

[mmenozzi]

Currently if someone wants to add value handlers to the Webgriffe\SyliusAkeneoPlugin\PriorityValueHandlersResolver has to redefine the whole webgriffe_sylius_akeneo.product.value_handlers_resolver and the friendly configuration at webgriffe_sylius_akeneo.value_handlers is not used anymore.

It should be easier to add more value handlers.

One solution could be to enrich the dependency injection extension to automatically inject value handlers inside the webgriffe_sylius_akeneo.product.value_handlers_resolver with a specific value handler tag.

[mmenozzi]

Done in #16. Closing.