We have an open issue at github for PGP support, and it's something I definitely want to pursue. The big concern is private key security and how to balance that with usability.
Is there a way to hook into keybase? https://keybase.io/ This project looks great, I use Fastmail but would like to have an IMAP web front end I host (I used to host my own stack), so I may give this a go. Thanks for sharing it!
Many mainstream (read: Apple/Microsoft) mail clients need plugins (which eg on iOS aren't an option) for PGP Mail, but S/MIME is handled out of the box.