Problem
Much of the code is written in old style es5
But node 6+ already supports es2016 and up natively

Describe the solution you'd like
Convert all the code by hand into es2016

Describe alternatives you've considered

• Convert all code to es2016+ and use babel to transpile the code down to es2016
• Convert all code to TypeScript and compile the code down to es2016
  TypeScript has the advantage of even better code optimization and documentation

Additional context
I have already started some successful experiments in this branch https://github.com/Shinigami92/HTMLHint/tree/experimental/babel-with-es2016
Also this branch is already a PR (#276): https://github.com/Shinigami92/HTMLHint/tree/refactoring/test/upgrade-to-es2016

node support of ES201X