Make it possible to seamlessly encrypt ement.el.gpg files on session save

[daviwil]

This change came out of a discussion with @alphapapa where we were trying to figure out how to encrypt the Ement session file with the least amount of work. This change was necessary to avoid a GPG key prompt every time the ement.el.gpg file gets saved.

[alphapapa]

Thanks. I'm slightly reluctant to add this, but I probably will. :)

[daviwil]

No worries if you're hesitant to! The alternative is to provide a hook at the same location where someone can configure the ement.el buffer before it gets saved, then I can inject whatever behavior I want.