Migrate to ES6

[tb]

I started a https://github.com/tb/html2jade/tree/es6 branch where I did setup of ES6 migration.

Please have a look and let me know if I can continue on it.

[donpark]

Forks are always welcome but conversion from CoffeeScript to ES6 won't add much value so I'd like to encourage you to try replacing the DOM-based conversion to HTML parser level conversion while you're at it.