When in `.editor` mode, current line whitespace prefixes
are preserved in the subsequent line. User can hit backspace
to clean the whitespace
```js
node 🙈 ₹ node
> .editor
// Entering editor mode (^D to finish, ^C to cancel)
function test() {
console.log('tested!'); //On enter, cursor will be after 2 spaces
_
```
PR-URL: https://github.com/nodejs/node/pull/8241
Reviewed-By: James M Snell <jasnell@gmail.com>
5bef38b627