feature: Open new buffer/window on new conversation

[KwisatzHaderach]

When I get a message from contact that is not in my opened buffers (weechat) I don't get any notification or imo better would be to open new buffer/window with that hangout.

[okurz]

That is solved by #15 so either also ask mtomwing to merge it or use https://github.com/okurz/pickups/tree/feature/auto_join for now