Implement 'R' for replace mode

[FazleArefin]

Pressing R in command mode will place the editor in replace mode for overwriting existing characters.

[jacekkopecky]

There's a PR #573 for this, but it got no comment from the maintainers.

[FazleArefin]

Great! Hope the maintainers notice this and does something about your PR #573

[gittyupagain]

+1

[bronson]

This is a dupe of #562. And I really do wish #573 would be merged or commented on.

[lurimendes]

+1

[tangledhelix]

Replace mode was merged some time ago - this issue can be closed, I think.