Add option to specify fixtures

[Hounddog]

Currently we can only specify a Directory path to the fixtures. We should also be able to specify each fixture as single entities.

[juriansluiman]

+1. And like the Symfony DoctrineFixturesBundle, an option to specify only a specific fixture or fixture directory would be nice too: --fixtures=/path/to/fixture.

This way you can point to specific classes in the configuration and at runtime also load fixtures from for example only one module.