Allow to override the addresses used to connect to an IRC network

Currently, biboumi derives the host name to use to connect to an IRC network from the JID, like this: #channel%hostname@biboumi.example (it uses the hostname part).

Normally, this is fine, since IRC networks provide well-known host names with DNS round-robin, possibly geolocated etc. magic to provide each client with a working server.

However, sometimes servers and/or networks are weird and it’s not possible to connect to some servers of the network, greatly deteriorating user experience. In that case, the classic resort is to find a server for a network which "just works" and uses that. With biboumi, instead of e.g. #foo%irc.somenetwork.net@… (with irc.somenetwork.net being the well-known entry point of somenetwork) we would now use #foo%some.server@… (with some.server being an IRC server which is part of somenetwork.

Now if some.server breaks down, we have the issue that to change to a different server (while staying on the same network), we would have to change all JIDs we use on the client side. That is inconvenient.

A possible solution would be to allow to specify a list of host names and/or IP addresses for biboumi to connect to via the Ad-Hoc interface on irc.somenetwork.net@…. We would then have used that to configure some.server as the only option to connect to irc.somenetwork.net; if that server fails, we can easily exchange it for another working server.

Compatibility: In the absence of such a configuration, biboumi could resort to the current way of doing things.

A next step would be to pre-configure well-known IRC networks with their entry points like this:

• ircnet@biboumi.example -> open.ircnet.net
• oftc@biboumi.example -> irc.oftc.net
• ...