Ctrl+W support for the REPL

FWIW, command-line style (delete back to whitespace) would be:
    leading = leading.replace(/\S+\s*$/, '');

lib/readline.js

@@ -331,6 +331,17 @@ Interface.prototype._ttyWrite = function (b) {
       this._historyNext();
       break;
 
+    case 23: // control-w, delete backwards to a word boundary
+      if (this.cursor !== 0) {
+        var leading = this.line.slice(0, this.cursor);
+        var match = leading.match(/\s?((\W+|\w+)\s*)$/);
+        leading = leading.slice(0, leading.length - match[1].length);
+        this.line = leading + this.line.slice(this.cursor, this.line.length);
+        this.cursor = leading.length;
+        this._refreshLine();
+      }
+      break;
+
     case 9: // tab, completion
       if (this.completer) {
         this._tabComplete();