Add placeholder attribute to text widgets

IPython/html/static/widgets/js/widget_string.js

@@ -141,6 +141,16 define(["widgets/js/widget"], function(WidgetManager){
                 .appendTo(this.$el);
             this.$el_to_style = this.$textbox; // Set default element to style
             this.update(); // Set defaults.
+            this.model.on('change:placeholder', function(model, value, options) {
+                this.update_placeholder(value);
+            }, this);
+        },
+
+        update_placeholder: function(value) {
+            if (!value) {
+                value = this.model.get('placeholder');
+            }
+            this.$textbox.attr('placeholder', value);
         },
         
         update: function(options){

IPython/html/widgets/widget_string.py

@@ -23,6 +23,7 class _StringWidget(DOMWidget):
     value = Unicode(help="String value", sync=True)
     disabled = Bool(False, help="Enable or disable user changes", sync=True)
     description = Unicode(help="Description of the value this widget represents", sync=True)
+    placeholder = Unicode("", help="Placeholder text to display when nothing has been typed", sync=True)
 
 
 class HTMLWidget(_StringWidget):