refactor tests and babel config. Fixes target browsers.

This is basically the change propagated from the current
default preact tempalate. Now the code compiles according to set
browsers.

tests/__mocks__/browserMocks.js

@@ -0,0 +1,21 @@
+// Mock Browser API's which are not supported by JSDOM, e.g. ServiceWorker, LocalStorage
+/**
+ * An example how to mock localStorage is given below 👇
+ */
+
+/* 
+// Mocks localStorage
+const localStorageMock = (function() {
+	let store = {};
+
+	return {
+		getItem: (key) => store[key] || null,
+		setItem: (key, value) => store[key] = value.toString(),
+		clear: () => store = {}
+	};
+
+})();
+
+Object.defineProperty(window, 'localStorage', {
+	value: localStorageMock
+}); */