Change up/down keys to cycle through history with currently typed prefix

I propose "searchForPrefix" to be the default behaviour for the up and down arrow keys.

-If you press up/down without typing anything, this is the same as before, as far as I can tell
-If you press up/down after typing, this behaviour is typically the behaviour you want
-If you press up/down after typing, and want the old behaviour, it is quick to kill the line and press up again
with C-a-k  or C-S-backspace in emacs mode, S in vi mode, or Ctrl+C in ghci, which is probably the vast 
majority of uses of ghci.

I get that this might be too big of a change from existing behaviour, but if you won't accept this patch, would you accept one that lets the user change the behaviour of the up/down keys in their settings file? That is what readline does.