Format src files with Prettier (#215)

Formats files in `src` with [prettier for consistent code style](https://prettier.io/docs/en/why-prettier) and adds npm script

Related to https://github.com/webrecorder/replayweb.page/issues/201

.eslintrc.js

@@ -3,7 +3,10 @@ module.exports = {
         "browser": true,
         "es6": true
     },
-    "extends": "eslint:recommended",
+    "extends": [
+        "eslint:recommended",
+        "prettier"
+    ],
     "parserOptions": {
         "ecmaVersion": 2018,
         "sourceType": "module"
@@ -13,21 +16,9 @@ module.exports = {
             2,
             "event", "error"
         ],
-        "indent": [
-            "error",
-            2
-        ],
         "linebreak-style": [
             "error",
             "unix"
-        ],
-        "quotes": [
-            "error",
-            "double"
-        ],
-        "semi": [
-            "error",
-            "always"
         ]
     }
 };