Add Memory Hole support
Open, NormalPublic

Description

Memory Hole is a project to move mail headers into the encytped/signed part of the mail.

this a good thing from privacy, we should support this.

https://github.com/autocrypt/memoryhole
https://wiki.gnupg.org/OpenPGPEmailSummit201607/MemoryHole

The basic idea is to have a mimelayout like this:

encrypted (after decryption):

multipart/encrypted:
    multipart/mixed
        text/rfc822-headers
        multipart/alternative
            text/plain
            text/html

and the text/rfc822-headers will have the encrypted ( and signed) approved headers of an mail like To,CC,BCC, Subject,References,...