Addresses issue #11 items unit testing, snapshot testing. Follows instructions from https://www.gatsbyjs.com/docs/how-to/testing/unit-testing/ ; Adding typescript config, mocks, snapshot capabilities; initial test for j40footer and snapshot, readme update, test helper for intl plugin (#79)

client/jest-preprocess.js

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