add functional warning

Author: yyx990803

lib/loader.js

@@ -210,6 +210,15 @@ module.exports = function (content) {
         '  }\n' +
         '})()}\n'
     }
+    // check functional
+    if (!isProduction) {
+      output +=
+        'if (__vue_options__.functional) {console.error("' +
+          '[vue-loader] ' + fileName + ': functional components are not ' +
+          'supported and should be defined in plain js files using render ' +
+          'functions.' +
+        '")}\n'
+    }
     // final export
     output += '\nmodule.exports = __vue_exports__ || __vue_options__\n'
   } else {