Upgrade to use es6 language features?

[nigelmegitt]

It'd improve code readability and conciseness if we could use ES6 language features. If there's an ongoing requirement to support engines that don't support them, perhaps we could add a build step to convert back to older language versions.

[BasBastian]

I am in progress of migrating the codebase with my own fork. Please feel free to share your opinion on the PRs quality

#228

I'll also try to squeeze that issue of "Set()" className collision