[placeholder plugin] Use editor direction

Closes #5174

addon/display/placeholder.js

@@ -38,6 +38,7 @@
     clearPlaceholder(cm);
     var elt = cm.state.placeholder = document.createElement("pre");
     elt.style.cssText = "height: 0; overflow: visible";
+    elt.style.direction = cm.getOption("direction");
     elt.className = "CodeMirror-placeholder";
     var placeHolder = cm.getOption("placeholder")
     if (typeof placeHolder == "string") placeHolder = document.createTextNode(placeHolder)