Add code coverage reports npm script

Generate code coverage reports based on the node-qunit tests with
istanbul.

Changes:
* Update README with npm run coverage
* Add .istanbul.yml to configure istanbul
* Add npm script "coverage" that runs istanbul and generates the reports

Example report: http://popups-coverage.surge.sh/lcov-report/index.html

Change-Id: I9be8c04b858a3ce6f4e29af2685b79253e3b4dca

.gitignore

@@ -1,4 +1,5 @@
 /docs/
+/coverage/
 /node_modules/
 /vendor/
 /composer.lock

.istanbul.yml

@@ -0,0 +1,8 @@
+instrumentation:
+  include-all-sources: true
+  excludes: [
+    "resources/dist/*", # Compiled assets
+    "*.js", # Gruntfile.js and webpack.config.js
+    "src/processLinks.js", # Covered by browser QUnit integration test
+    "src/index.js", # Application entry point
+  ]

README.md

@@ -31,3 +31,5 @@ After an `npm install`:
     * `npm install -g nodemon`
     * Example running linting and node unit tests:
       * `nodemon -w src/ --exec "grunt lint:all && npm run test:node"`
+  * Get code coverage report with `npm run coverage`
+    * Reports printed in the `coverage/` folder

package.json

@@ -6,7 +6,8 @@
     "test": "grunt lint && npm run check-built-assets && npm run test:node",
     "doc": "jsduck",
     "check-built-assets": "echo 'CHECKING BUILD SOURCES ARE COMMITED' && rm -rf test-build && mv resources/dist test-build && npm run build && diff -x '*.map' -r test-build resources/dist && rm -rf test-build",
-    "test:node": "mw-node-qunit tests/node-qunit/*.js tests/node-qunit/**/*.js | tap-dot"
+    "test:node": "mw-node-qunit tests/node-qunit/*.js tests/node-qunit/**/*.js | tap-dot",
+    "coverage": "istanbul cover node_modules/.bin/mw-node-qunit tests/node-qunit/*.js tests/node-qunit/**/*.js"
   },
   "devDependencies": {
     "eslint-config-wikimedia": "0.3.0",
@@ -18,8 +19,9 @@
     "grunt-eslint": "19.0.0",
     "grunt-jsonlint": "1.1.0",
     "grunt-stylelint": "^0.6.0",
-    "mw-node-qunit": "^1.0.6",
+    "istanbul": "^0.4.5",
     "mock-require": "^2.0.1",
+    "mw-node-qunit": "^1.0.6",
     "redux": "3.6.0",
     "redux-thunk": "2.2.0",
     "stylelint-config-wikimedia": "0.3.0",