Add npm script for HTML coverage reports

Author: afk11

package.json

@@ -36,6 +36,7 @@
  ],
  "scripts": {
  "coverage-report": "nyc report --reporter=lcov",
+ "coverage-html": "nyc report --reporter=html",
  "coverage": "nyc --check-coverage --branches 90 --functions 90 mocha",
  "integration": "mocha test/integration/",
  "standard": "standard",