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

Builds off PR #26.

* Adds Jest/Enzyme and the first part of a simple React component test, which can be executed via 'npm run test'.
* Adds eslint rules for Jest
* Adds new devDeps:
  * babel-jest: Compiles JS code using Babel in Jest tests
  * babel-preset-env: Compiles ES2015+ down to ES5. Currently this is so we can use 'import' in Node.js for the Jest tests.
  * enzyme: JS testing library for React
  * enzyme-adapter-react-16: Allows Enzyme to be compatible with React 16
  * eslint-plugin-jest: Expose Jest-specific rules for use in ESLint
  * jest: Testing framework for JS including React apps
  * react-test-renderer: A peer dependency for enzyme-adapter-react-16. An experimental React renderer that can be used to render React components to pure JS objects, without depending on the DOM or a native mobile environment.
  * regenerator-runtime
* Adds a 'commerce' alias for Jest tests in 'package.json' for React components to match the 'commerce' alias used in Webpack.
* Adds static asset mocks for Jest tests
* Export both 'Accordion' and 'AccordionSection' classes from 'Accordion.jsx' rather than just 'Accordion', so that 'AccordionSection' can also be used in Jest tests.

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}],