Add support for optional service referencesin YAML services configuration

[FlyingDR]

Symfony have support for special syntax for defining optional service references, but currently (in v0.10.34) there seems to be no support for them.

When defining service as:

my_service:
  class: My\Service
  arguments: [@doctrine.orm.entity_manager, @?logger]

PHPStorm 7.0 reports of "Missing service" at @?logger, problem report disappears after removing ? from reference definition.

[Haehnchen]

its fixed on our side. but some error on intellj. see external issue http://youtrack.jetbrains.com/issue/IDEA-118077