Add a performance disclaimer to the banner

I sincerely believe that adding this disclaimer (or one like it) to the start of every interactive Julia session will substantially decrease the confusion that new Julia users experience. Obviously this is a last-resort kind of measure, but we've had the same performance problems being reported for years. I would argue that the lack of optimization in the global namespace is the single easiest thing newcomers can complain about it, since it makes Julia's fabled C-like performance seem like a lie.

@stevengj also noted on julia-users that we could add this behavior to `toc`, although I couldn't tell if he was being sarcastic or not.