fish vi-mode: Implement replace & overwrite modes
fish vi-mode: Implement replace & overwrite modes

The overwrite mode is not working, but it's not clear why as the replace
mode uses essentially the same code and seems to work just fine.

Signed-off-by: Ian Munsie <ce389fd2fa887d610219cb85dea6dc9451973e74@gmail.com>

DarkStarSword/junk,DarkStarSword/junk,DarkStarSword/junk,DarkStarSword/junk,DarkStarSword/junk