jabber: set server domain manually

[ilf]

The Jabber protocol handler seems to take the server to connect to from the domainpart of the jid: ​https://tools.ietf.org/html/rfc7613#section-6 ​https://github.com/bitlbee/bitlbee/blob/master/protocols/jabber/jabber.c#L125

In most cases, this is good. However, in some cases, it is desirable to set a different server addess manually, f.e.:

• a different domain, f.e. xmpp.example.com instead of example.com (vor certificate validation purposed)
• an IP-address instead of a domain (to remove the layer of DNS)
• a Tor Hidden Service .onion

Therefore, I propose to add an optional setting "server" per jabber-account to override the server manually.