readline: fix for unicode prompts

prompt length is char length, not byte length

lib/readline.js

@@ -136,7 +136,7 @@ Interface.prototype.setPrompt = function(prompt, length) {
   } else {
     var lines = prompt.split(/[\r\n]/);
     var lastLine = lines[lines.length - 1];
-    this._promptLength = Buffer.byteLength(lastLine);
+    this._promptLength = lastLine.length;
   }
 };