borgmatic 1.5.15

• #419: Document use case of running backups conditionally based on laptop power level:
  https://torsion.org/borgmatic/docs/how-to/backup-to-a-removable-drive-or-an-intermittent-server/
• #425: Run arbitrary Borg commands with new "borgmatic borg" action. See the documentation for
  more information: https://torsion.org/borgmatic/docs/how-to/run-arbitrary-borg-commands/