ENH: Add support for encrypting mail

[millerthegorilla]

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I would like to be able to encrypt the contents of my mail, I would also like to be able to encrypt my password. Adding an API to redmail that would allow this to happen would be really good.

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Add API that offers a simple interface to encryption

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python-gnupg with gpg-agent (on a headless box, so no gnome etc), keyring in desktop environment

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Whilst an encryption password might be necessary per email sent, it would be perfect if the encryption could be run non-interactively, obtaining the cleartext passphrase from a protected, audited file, or from the gpg keyring itself.