Add visual regression test and code coverage report

[compulim]

Added

• Development build now include instrumentation code, updated build scripts
  • npm run build will build for development with instrumentation code
  • npm run prepublishOnly will build for production
  • npm run watch will also run Webpack in watch loop
• Automated testing using visual regression testing technique
  • Docker-based automated testing using headless Chrome and Web Driver
  • Screenshot comparison using jest-image-snapshot and pixelmatch
  • Code is instrumented using istanbul
  • Test report is hosted on Coveralls

[compulim]

Self-approving non-production code.