Use Prettier for consistent code formatting

package.json

@@ -5,8 +5,11 @@
   "repository": "git+https://github.com/tyrasd/overpass-turbo.git",
   "license": "MIT",
   "scripts": {
-    "test": "karma start --single-run",
+    "test": "npm run test:style && npm run test:karma",
+    "test:style": "prettier --no-bracket-spacing --list-different $(git ls-files '*.js')",
+    "test:karma": "karma start --single-run",
     "test:start": "karma start",
+    "fix:style": "prettier --no-bracket-spacing --write $(git ls-files '*.js')",
     "start": "webpack-dev-server",
     "build": "webpack -p",
     "deploy": "npm run build && gh-pages -d build"
@@ -33,6 +36,7 @@
     "pegjs": "^0.10.0",
     "pegjs-loader": "^0.5.1",
     "phantomjs-prebuilt": "^2.1.14",
+    "prettier": "^1.3.1",
     "request": "^2.67.0",
     "sinon": "^2.0.0-pre.5",
     "sinon-chai": "^2.8.0",