XMPP PubSub support (XEP-0060)

[sjehuda]

Similar to #535

Allow sharing links via PubSub node items.

Proposed node to store bookmarks: urn:xmpp:bibliography:0.

[sjehuda]

Please see https://git.xmpp-it.net/sch/Blasta

Blasta is a collaborative bookmarking system which relies on XMPP.

It has an internal database to connect accounts with shared links.

I think that LDAP and XMPP would definitely make Bookie more relevant, popular and prevalent.