Rename displayItem to showItem to match 'show' command name
Omit empty field values from JSON content

This is consistent with items generated by 1Password for iOS
Implement trashing and deletion of items correctly

 * Add 'trash' and 'restore' commands to move items to/from trash

 * When deleting items, instead of removing the data files, erase
   their content and title and change their type to 'system.Tombstone'
   so that the deletion is synced to other 1Password clients