CKEDITOR.plugins.add("variables", {
  init: function(editor) {
    console.log("variables.init")

    editor.addCommand("insertVariable", {
     allowedContent: "span[contenteditable](*)"
     ,requiredContent: "span[contenteditable](*)"
     ,exec: function (editor) {
       console.log("insertVariable.exec");
       editor.insertElement(CKEDITOR.dom.element.createFromHtml('<span class="variable" contenteditable="false">[ variable ]</span>'));
     }
    })

    editor.ui.addButton("InsertVariableButton", {
     label: "Variable"
     ,title: "Insert variable"
     ,toolbar: "others"
     ,command: "insertVariable"
    })
  }
})