Start to add basic encryption support.

At this time, GnuPG is the only supported encryption method.  OpenSSL support
is on the way.

The GnuPG decryptor supports both binary and ascii-armored PGP-encrypted
files.

To use GnuPG encryption:

    fresh config/dotfile.gpg --crypt=gpg
   fresh config/dotfile.asc --crypt=gpg