Add npm script for prettier

package.json

@@ -40,6 +40,7 @@
     "test:firefox": "lerna run test:firefox --scope \"@jupyterlab/test-*\" --concurrency 1 --stream",
     "test:ie": "lerna run test:ie --scope \"@jupyterlab/test-*\" --concurrency 1 --stream",
     "test:services": "cd packages/services && jlpm test && cd examples/node && python main.py",
+    "prettier": "prettier --write '**/*{.ts,.tsx,.js,.jsx,.css,.json,.md}'",
     "update:dependency": "node buildutils/lib/update-dependency.js",
     "watch": "run-p watch:dev watch:themes",
     "watch:dev": "python scripts/watch_dev.py",