chore: Add prettier (#467)

Author: wangyiz4262

package.json

@@ -9,6 +9,7 @@
     "test": "TZ=Pacific/Auckland npm run test-tz && TZ=Europe/London npm run test-tz && npm run test-tz && jest",
     "test-tz": "jest test/timezone.test --coverage=false",
     "lint": "./node_modules/.bin/eslint src/* test/* build/*",
+    "prettier": "prettier --write \"{src,test,build,docs}/**/*.{js,ts,md}\"",
     "babel": "cross-env BABEL_ENV=build babel src --out-dir esm --copy-files",
     "build": "cross-env BABEL_ENV=build node build && npm run size",
     "sauce": "npx karma start karma.sauce.conf.js",
@@ -86,6 +87,7 @@
     "mockdate": "^2.0.2",
     "moment": "^2.22.0",
     "pre-commit": "^1.2.2",
+    "prettier": "^1.16.1",
     "rollup": "^0.57.1",
     "rollup-plugin-babel": "^4.0.0-beta.4",
     "rollup-plugin-uglify": "^3.0.0",

prettier.config.js

@@ -0,0 +1,9 @@
+module.exports = {
+  useTabs: false,
+  printWidth: 80,
+  tabWidth: 2,
+  singleQuote: true,
+  trailingComma: 'none',
+  semi: false,
+  arrowParens: 'avoid'
+}