Added support for preserving html comments in generated html output through env value

Created by: ryanmcafee

Added support for preserving html comments in the generated html file to support Enterprise use case of using server side includes with Nginx, Apache, etc.. Verified by setting ENABLE_HTML_COMMENTS to true in .env file and running: npm run build observing that html comments were preserved in build/index.html. Verified by setting ENABLE_HTML_COMMENTS to false in .env file and running: npm run build observing that html comments were removed in build/index.html. Updated advanced configuration documentation with additional configuration option. Preserved existing default behavior as to not impact existing applications.