There are a number of options that can be added to this application to make it
more flexible. Here are some ideas that I will implement when I get around to
it. Patches welcome.

1.) Use Key IDs interchangeably with email addresses

2.) Optionally support signing as well as encrypting.

  --sign-key       "The email address or key id to sign with"
  --sign-pass      "The password for the secret signing key"
  --sign-pass-file "Path to file containing password for the secret signing key"

3.) If there are no recipients, should we just output the original message and
    exit, or die?

  --on-missing-recipients output-plain / die

4.) Perhaps have an optional default recipient option which is used if no other
    recipients are provided.

  --default-recipient     key-id or email-address

5.) If one or more of the recipients doesn't have a public key in the local gpg
    ring how do we behave?

  --on-missing-key        output-plain  / discard-recipient / die

6.) At the moment, we prefer inline encryption and fall back to PGP/MIME if
    the message is multipart. Should provide several different optional modes
    of encryption.

  --encrypt-mode          prefer-inline / pgpmime / inline-or-plain

7.) Optionally specify path to the gpg "homedir" on the command line

  --gpg-home              path-to-gnupg-directory