Add feature to enforce licence metadata

Add a feature to enforce that skills have a licence file and that the licence metadata matches a list of approved licences.
Will include
* A new command line flag to enforce the presence of the `licence` metadata entry in frontmatter.
* A new command line flag to pass a list of approved licence files that the metadata entries must match.
* A new command line flag to print deduplicated list of licence values for all the skills analyzed
* A new yaml config entry for enforcing licence metadata presence
* A new yaml config entry for enforcing that the licence entry matches one of a list of approved licence files