Adding Jest for testing

jest/preprocess.js

@@ -0,0 +1,5 @@
+const babelOptions = {
+  presets: ['babel-preset-gatsby']
+};
+
+module.exports = require('babel-jest').createTransformer(babelOptions);

jest/setup.js

@@ -0,0 +1,8 @@
+import Enzyme from 'enzyme';
+import Adapter from 'enzyme-adapter-react-16';
+
+Enzyme.configure({ adapter: new Adapter() });
+
+global.___loader = {
+  enqueue: jest.fn()
+};