Add vi mode indicator

Readline has an option: show-mode-in-prompt
that works with `editing-mode vi`
to give an indicator of the vi editing mode, in the form of the leftmost prompt character showing a '+' in insert mode, and ':' in normal mode, (it removes the character all-together when doing a search with /, I guess if it supports other command-mode commands, it would do the same thing)

It would be great if I could get this in my ghci..
That being said, just because readline does it this way, doesn't mean it makes more sense than having something customizable in the configuration of the prompt itself.
Any thoughts?