- require age.el if the package is installed
  https://github.com/anticomputer/age.el
- add new customizable variable `org-journal-encryption-extension` to
  control the file extension appended to the file name to trigger
  transparent encryption
- recognize `.age` files as journal files
- generate encrypted files based on chosen file extension instead of
  hard coding `.gpg`
- support `.age` files in `org-journal-re-encrypt-journals`
    - only re-encrypt files for the currently chosen encryption
      extension
    - functional change: don't edit/re-save files that aren't
      encrypted. I assume that they where handled by this function in
      the past was an oversight and not intended.
    - allow multiple recipients for `age` encryption

closes bastibe#441