Add support for tags

[Duncan McGreggor]

Tasks:

• Add tags field to Metadata
• Add logic for basic operations on tags
• Implement set logic for tests (and/or) in util module
  • for any, use intersection == non-zero; https://doc.rust-lang.org/std/collections/hash_set/struct.Intersection.html
  • for all, ensure input is a subset; https://doc.rust-lang.org/std/collections/hash_set/struct.HashSet.html#method.is_subset
• Add --all-tags flag (AND)
• Add --any-tags flag (OR)
• Add show tags subcommand

[Duncan McGreggor]

Done.