0.5.0

• Add support for multiple values under same key
  • Add Tag::items() method.
  • Add Tag::add_item() method.
  • Replace Tag::remove_item() by Tag::remove_items() method.
    Tag::item() method returns a first found item.
    Tag::set_item() removes all items under the given key and adds a new one.
• Derive Clone for Item and ItemValue structs.