Add prettier config, format script and reformat files

Summary:
Prettier 1.7.0 has config file + pragma support so this works really well with the current prettier usage. Also added a prettier script to run it (taken mostly from the relay repo) and ran it which caused 1 files to change (probably wasn't updated when upgrading prettier to 1.7.0).

**Test plan**
Made sure flow still checked
Run 'yarn prettier', should format only files with `format` using the config in package.json.
Closes https://github.com/facebook/react-native/pull/16176

Differential Revision: D6256899

Pulled By: shergin

fbshipit-source-id: 646d90c15db8032b7b551da228d26d370babf125

package.json

@@ -10,6 +10,14 @@
   "engines": {
     "node": ">=4"
   },
+  "prettier": {
+    "requirePragma": true,
+    "singleQuote": true,
+    "trailingComma": "all",
+    "bracketSpacing": false,
+    "jsxBracketSameLine": true,
+    "parser": "flow"
+  },
   "jest": {
     "transform": {
       "^[./a-zA-Z0-9$_-]+\\.(bmp|gif|jpg|jpeg|mp4|png|psd|svg|webp)$": "<rootDir>/jest/assetFileTransformer.js",
@@ -118,6 +126,7 @@
     "test": "jest",
     "flow": "flow",
     "lint": "eslint .",
+    "prettier": "find . -name node_modules -prune -or -name '*.js' -print | xargs prettier --write",
     "start": "/usr/bin/env bash -c './scripts/packager.sh \"$@\" || true' --",
     "test-android-setup": "docker pull containership/android-base:latest",
     "test-android-build": "docker build -t react/android -f ContainerShip/Dockerfile.android .",