Add an easy way to run test suite once

Author: meiamsome

package.json

@@ -13,6 +13,7 @@
     "start": "webpack-dev-server --open --config webpack.dev.babel.js",
     "build": "webpack --config webpack.prod.babel.js",
     "test": "./node_modules/karma/bin/karma start karma.conf.js",
+    "test:single": "./node_modules/karma/bin/karma start karma.conf.js --single-run",
     "test-travis": "./scripts/test-travis.sh"
   },
   "repository": {