config: get or set global options

Usage: akt config name value

  Configure a global option for akt.  The following configuration
  properties are recognized:
  
  gpg-encrypt     GPG2 command to run to encrypt a secret
  gpg-decrypt     GPG2 command to run to decrypt a secret
  gpg-list-keys   GPG2 command to run to get the available private keys
  fill-zero       When set to 'yes' fill space with zero instead of random

