#3 Optional data encryption

[Steve Rosen]

Provide an option to encrypt the backup archive to
provide data security. The encryption should allow for
decryption at the command line.

[Steve Rosen]

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This change would impact several areas of the design:

- User interface: Provide an option for encrypting the
backup file and a field to collect the passphrase
- Config file: Encryption option must be added to the
configuration file, and possibly the name of a file
containing the passphrase
- Passphrase file: A new file with limited access (maybe
600) containing the passphrase
- Update to the bkup script to read and use the encryption
option