borgmatic 1.8.2

• #345: Add "key export" action to export a copy of the repository key for safekeeping in case
  the original goes missing or gets damaged.
• #727: Add a MariaDB database hook that uses native MariaDB commands instead of the deprecated
  MySQL ones. Be aware though that any existing backups made with the "mysql_databases:" hook are
  only restorable with a "mysql_databases:" configuration.
• #738: Fix for potential data loss (data not getting restored) in which the database "restore"
  action didn't actually restore anything and indicated success anyway.
• Remove the deprecated use of the MongoDB hook's "--db" flag for database restoration.
• Add source code reference documentation for getting oriented with the borgmatic code as a
  developer: https://torsion.org/borgmatic/docs/reference/source-code/