repl: Auto alignment for .editor mode

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>

1 files changed, 8 insertions, 0 deletions

diff --git a/lib/readline.js b/lib/readline.js
index f7591b7cc1..e45fb2938e 100644
--- a/lib/readline.js
+++ b/lib/readline.js
@@ -41,6 +41,7 @@ function Interface(input, output, completer, terminal) {
 
   this._sawReturn = false;
   this.isCompletionEnabled = true;
+  this._sawKeyPress = false;
   this._previousKey = null;
 
   EventEmitter.call(this);
@@ -247,6 +248,9 @@ Interface.prototype._addHistory = function() {
   // if the history is disabled then return the line
   if (this.historySize === 0) return this.line;
 
+  // if the trimmed line is empty then return the line
+  if (this.line.trim().length === 0) return this.line;
+
   if (this.history.length === 0 || this.history[0] !== this.line) {
     this.history.unshift(this.line);
 
@@ -947,6 +951,10 @@ function emitKeypressEvents(stream, iface) {
       if (r) {
         clearTimeout(timeoutId);
 
+        if (iface) {
+          iface._sawKeyPress = r.length === 1;
+        }
+
         for (var i = 0; i < r.length; i++) {
           if (r[i] === '\t' && typeof r[i + 1] === 'string' && iface) {
             iface.isCompletionEnabled = false;