Add `backup` subcommands

[Duncan McGreggor]

Tasks:

• Add a new backup command to the CLI
  • with no additional subcommands, perform a backup
    • for a non-daemonised instance, this is just a file copy
    • for a daemonized instance, this is a request issued to the server -- this depends upon #41
    • update #41 with this enhancement task
• add a new backup list command so show all (including timestamp)
• add backup list --latest to show the most recent backup only
• add a new backup restore command (by index or filename)
• add alias list backups to backup list
• add show backup-dir subcommand
• add a new backup delete command
• refactor file utils into their own module
• Add example docstrings for all:
  • backup list
  • list backups
  • backup list --latest
  • backup restore
  • show backup-dir
  • backup delete