Add support for JMAP (JSON Meta Application Protocol), as an alternative to IMAP + SMTP

[marclaporte]

"JMAP is intended to be a new standard for email clients to connect to mail stores. It therefore intends to primarily replace IMAP + SMTP submission. It is also designed to be more generic such that it can be extended with contacts, calendars in the future (replacing CardDAV/CalDAV). It does not replace MTA-to-MTA SMTP transmission."

The 3 main Open Source servers that support it:

• Cyrus IMAP: https://github.com/cyrusimap/cyrus-imapd
• Apache James: https://github.com/apache/james-project
• Stalwart: https://github.com/stalwartlabs/jmap-server

Support on the client-side: https://github.com/jmapio/jmap/blob/master/software/software.mdown#clients

Please also see:

• http://jmap.io/
• https://en.wikipedia.org/wiki/JSON_Meta_Application_Protocol
• https://jmap.io/spec.html

Thanks!