loader class for Data::Phrasebook using YAML

 Data::Phrasebook::Loader::YAML implements phrasebook patterns using YAML.
 .
 Phrases can be contained within one or more dictionaries, with each phrase
 accessible via a unique key. Phrases may contain placeholders, please see
 Data::Phrasebook for an explanation of how to use these. Groups of phrases
 are kept in a dictionary. In this implementation a single YAML file is one
 complete dictionary.