Allow configurable default config directory

Currency 7.x-2.x uses external libraries with Yaml files that are used as configuration files. The idea was that in Drupal 8, these files could be used as default configuration. However, they are not located in ./path/to/module/config/, but instead in ./path/to/module/vendor/.... By adding a property to .info files, we can tell the system to look for config in a different directory.