mcabber: don't show offline contacts by default

I can always turn it back on with /roster toggle_offline (bound to F5).
mcabber: hide roster by default

I don't really need it that much most of the time. I turn it off mainly
because it causes problems with tmux' activity monitor: mcabber
constantly updates statuses, tmux notices it and tells me there's some
activity in the window. But such an activity is of no interest to me.