[Feature request] Add support of MAM

[ghost]

Add support of XEP-0313: Message Archive Management.

https://xmpp.org/extensions/xep-0313.html

(It would be nice ;) )

[ghost]

There is an implementation on the mailing list:

• https://sourceforge.net/p/emacs-jabber/mailman/message/35603252/

It does have some flaws (sometimes it hangs, sometimes all messages are on one line, sometimes the datestamps are all NOW - I haven't reported these flaws, I have to admit), but it is a good start!