Add support for tags

[oubiwann]

Tasks:

• [x] Add tags field to Metadata
• [x] Add logic for basic operations on tags
• [x] 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
• [x] Add --all-tags flag (AND)
• [x] Add --any-tags flag (OR)
• [x] Add show tags subcommand

[oubiwann]

Done.