replace :extension directive with :flag.

I would prefer to use :set, since that's what ghci does,
and put the IHaskell-specific settings somewhere else.
Setting extensions this way `:flag -XExistentialQuantification`
means that -XExplicitForAll gets implied (#108 part 1).
The part 2 of that isn't addressed (yet).

The -package flag is broken for now. Also no completions.
do completion for :set flags and :flag flags