various config stuff in rcirc.el should be customizable

[Miles Bader]

They're now defvars, but they should be defcustoms e.g., `rcirc-nick', etc.

Thanks,

-miles

-- 
"An atheist doesn't have to be someone who thinks he has a proof that there
can't be a god.  He only has to be someone who believes that the evidence
on the God question is at a similar level to the evidence on the werewolf
question."  [John McCarthy]