Use prettier (#600)

scripts/yarn-install.js

@@ -0,0 +1,13 @@
+"use strict";
+
+const exec = require("child_process").exec;
+
+const runIfYarn = fn => {
+  exec("yarn -V", error => {
+    if (error === null) fn();
+  });
+};
+runIfYarn(() => {
+  console.log("`package.json` was changed. Running yarn...🐈");
+  exec("yarn");
+});