Change get_cellar method to fetch more metadata

[shashankmc]

Currently, additional metadata like keywords, case law directory code and eurovoc identifiers are part of get_cellar_extra method but such information are useful as metadata itself.

• [ ] Move parts of get_cellar_extra which fetch keywords, case_law_directory_code and eurovoc_identifiers to get_cellar method.
• [ ] Modify get_cellar_extra to only fetch full_text while calling the get_cellar for other metadata information.
• [ ] Update readme file
• [ ] Create issue tracker on downstream application (like case-law-explorer ETL)