add umd script setting

Author: 黄书伟

package.json

@@ -66,7 +66,8 @@
   "scripts": {
     "build": "webpack --env.build",
     "dev": "webpack-dev-server --env.dev",
-    "build:libs": "node build/build-libs.js"
+    "build:libs": "node build/build-libs.js",
+    "umd":"webpack --env.umd --config build/webpack.umd.config.js"
   },
   "bugs": {
     "url": "https://github.com/huangshuwei/vue-easytable/issues"