Add prettier

.prettierrc

@@ -0,0 +1,3 @@
+{
+  "singleQuote": true
+}

package.json

@@ -26,20 +26,21 @@
   "scripts": {
     "build": "tsc",
     "clean": "rimraf lib",
+    "prettier": "prettier --write 'src/**'",
     "watch": "tsc -w"
   },
   "dependencies": {
     "@jupyterlab/application": "^0.16.0",
     "@jupyterlab/apputils": "^0.16.4",
-    "@jupyterlab/mainmenu": "^0.5.3",
     "@jupyterlab/coreutils": "^1.1.3-1",
+    "@jupyterlab/mainmenu": "^0.5.3",
     "@phosphor/widgets": "^1.6.0",
     "react": "^16.4.1",
     "react-dom": "^16.4.1",
-    "tsc": "^1.20150623.0",
     "typestyle": "^2.0.1"
   },
   "devDependencies": {
+    "prettier": "^1.13.7",
     "rimraf": "^2.6.1",
     "typescript": "~2.6.0"
   },