Implement NPM scripts for multiple test tasks

This commit adds new NPM scripts to allow to run all tests with various
options:

- `test` - Run all tests with Jest.
- `test:cov` - Run all tests with coverage reports (1).
- `test:watch` - Run all tests in Jest's "watch mode" (2).

References:
  (1) https://jestjs.io/docs/en/cli#coverage
  (2) https://jestjs.io/docs/en/cli#watch

GH-39

package.json

@@ -38,7 +38,10 @@
     "lint": "npm-run-all lint:*",
     "lint:js": "eslint --ext .js,.jsx .",
     "lint:md": "remark --no-stdout . \".github/**/*.md\"",
-    "serve": "gatsby serve"
+    "serve": "gatsby serve",
+    "test": "jest",
+    "test:cov": "jest --coverage",
+    "test:watch": "jest --watch"
   },
   "devDependencies": {
     "@babel/core": "7.1.6",