[runmode addon] Define text/plain mode in standalone script.

addon/runmode/runmode-standalone.js

@@ -91,6 +91,10 @@ CodeMirror.getMode = function (options, spec) {
   return mfactory(options, spec);
 };
 CodeMirror.registerHelper = CodeMirror.registerGlobalHelper = Math.min;
+CodeMirror.defineMode("null", function() {
+  return {token: function(stream) {stream.skipToEnd();}};
+});
+CodeMirror.defineMIME("text/plain", "null");
 
 CodeMirror.runMode = function (string, modespec, callback, options) {
   var mode = CodeMirror.getMode({ indentUnit: 2 }, modespec);