Protect headers by using autocrypt memoryhole

[nebulon42]

Following up from #46.

It is beneficial to protect header information (subject, message id, etc) which are not normally encrypted by PGP. The memoryhole project part of autocrypt tries to provide a standardized way to do that, see https://github.com/autocrypt/memoryhole.

There is also a WIP Python library for the project which could be of use here: https://0xacab.org/leap/pymemoryhole

[nerds-R-bae]

Much support for this integration, thanks.

[inclementweather]

I'm experimenting with some changes that support this standard. I will open a pull request within the next few weeks after some testing.