Added a file-based configuration system

Drupal 8 introduced a new API to centralize the storage and accessing of configuration information. This replaces the existing system of variable_get()/variable_set() as well as many one-off storage systems like the ones used for image styles, and allows configurations to be imported and exported, as well as staged and synchronized between different instances of the same site. For detailed information, see the Configuration API in Drupal 8 documentation.