readline: make `ctrl + L` clear the screen

lib/readline.js

@@ -663,6 +663,12 @@ Interface.prototype._ttyWrite = function(s, key) {
         this._moveCursor(+1);
         break;
 
+      case 'l': // clear the whole screen
+        exports.cursorTo(this.output, 0, 0);
+        exports.clearScreenDown(this.output);
+        this._refreshLine();
+        break;
+
       case 'n': // next history item
         this._historyNext();
         break;