Allow fixtures tagging with TaggedFixtureInterface

This PR introduces ``TaggedFixtureInterface`` which allows you to tag your fixtures with tags using ``getTags`` method in fixture's class.

Use case: When using ``loadFromDirectory`` it would be nice to filter fixtures by something, e.g. tags.

Genesis: I'm going to update ``doctrine/data-fixtures-bundle`` and ``symfony/doctrine-bridge`` to allow developers tag fixtures. This will allow to have single fixtures directory in which you will put staging (or production) fixtures and development fixtures. Code updates for these two packages are already done (available on my github account) and will be used to create PRs one after another, as soon as this PR (if ever) gets merged.

The interface can be used with other interfaces without any regression.