Mostly add `julia-repl` for more specific highlighting.

- Removed the automatic preexecute hook, IJulia will soon do this automatically
  in all cases through a package extension.
- Removed the recommendation of using `startup_ijulia.jl`. This is not
  necessary since IJulia will already use Revise from `startup.jl` if it's
  set. And if Revise is loaded only through `startup_ijulia.jl` then IJulia will
  end up adding a duplicate preexecute hook.