Introduce the `copy` command

Added

• Added the new imap-backup copy command to replace the existing
  migrate and mirror commands. The new command is simpler
  than the exiting commands, while maintaining the principal functionality
  that was previously available. migrate and mirror are now deprecated
  and will be removed in a future release.

Changed

• The library now uses logger and ostruct as external dependencies
  as they have been extracted from the Ruby standard library.