Consider using ":add" in generated ghci script

[mgsloan]

Like #1886 , but for all modules. The main advantage of this is potentially working better with the user's ghci scripts. I don't recall if the user's script comes before or after ours. But either way, :add would allow user scripts to augment the list of loaded modules.

[mgsloan]

I figure this is a good thing to add - committed.