Add a config setting to make wfd_meta-based caches expire immediately (#28)

This release adds a new configuration setting:

# config_test.yml
webfactory_wfd_meta:
    always_expire_wfd_meta_resources: true

With this setting, ConfigCache instances (Symfony Router, Translator, ... maybe?) that include WfdMetaResource instances will expire immediately.

This is helpful e. g. in functional tests, where you have database-backed routes, translations or similar: You can change the database values and no longer need to think about the ConfigCaches or poke wfd_meta to make the changes effective.