[install purple-plugin-delta] support out-of-box Email-chat spec communication

There is a libpurple plugin that supports (autocrypted) chatting over and with any standard email address (also backwards compatible with classic email client users). Repository: https://gitlab.com/lupine/purple-plugin-delta The plugin was recently updated and is now based on the current deltachat-core code.

Note that the deltachat development has already lead to Open-Xchange (OE) pushing the Email-chat standard to their email provider customers, and this has also been presented at Fosdem. https://www.coi-dev.org/ You can find and check out the current reference client implementations for Android (Play-Store and F-Droid), Desktop, node.js, and iOS (Testflight) through https://delta.chat