Adds @returns annotations.
Adds a Syllabus#domain method, which returns an ExtractResult from tldextract.
Adds parsed_domain and registered_domain methods on Syllabus.
Corrects the docblocks on Syllabus#parsed_domain / registered_domain.
Route the metadata getters through a Syllabus#metadata method, which returns None if the log isn't readable.
Uses a custom requires_attr method to harden up metadata getters on Syllabus.