YAML loading and player config schema

 - Renamed current `Yaml` function to `YamlFile` and moved it to package `provo.yaml`
 - Added new `Yaml` class, subclass of `YamlConfiguration`, that has methods to parse unnamed lists of dictionaries
 - `YamlFile` now uses said `Yaml` class
 - Fuck this is confusing
 - Added parsing code in `SortingPreferencesBackend` - tested
 - Added a default initialized `SortingPreferencesBackend` instance