Fix #28: WIP - Add initial Jest test for a React component.

* Adds Jest/Enzyme and the first part of a simple React component test.
* Adds a 'commerce' alias for Jest/Enzyme tests for React components to match the 'commerce' alias used in Webpack.
*

Questions:
* How to get around the CSS import in Accordion.jsx? I currently have it commented out.
  * Is it possible for me to webpack the './test' directory? Would I want to? What might that look like?
* How can I render the original '<Accordion />', instead of explicitly passing it two '<AccordionSection />' elements in the test? 'Accordion.jsx' renders two AccordionSections already with their own 'this.props.children'.
* How can I fix the prop-types errors for required props? Do I just have to 'monkey patch' these components and pass dummy props into the components in the test?
* Should I use 'babel-preset-env', so that I can use 'import' in the test files? These tests run in Node.js, and currently, Node.js v8 still does not support 'import'.
* Jest docs (https://jestjs.io/docs/en/webpack) suggest using mocks for static assets. I followed their Webpack instrucions for this, but I'm not sure I follow what the mock files are actually for/doing.

.babelrc

@@ -1,5 +1,8 @@
 {
-  "presets": ["react"],
+  "presets": [
+    "react",
+    "env"
+  ],
   "plugins": [
     "transform-class-properties",
     "transform-decorators-legacy"

.eslintrc.json

@@ -1,10 +1,17 @@
 {
-  "extends": "airbnb",
+  "extends": [
+    "airbnb",
+    "plugin:jest/recommended"
+  ],
   "env": {
     "webextensions": true,
-    "browser": true
+    "browser": true,
+    "jest/globals": true
   },
-  "plugins": ["react"],
+  "plugins": [
+    "react",
+    "jest"
+  ],
   "parser": "babel-eslint",
   "rules": {
     "object-curly-newline": ["error", {"consistent": true}],