Update code formatting

[elyas-bhy]

Is maintaining compatibility with Node 4.x a priority, or could we bump it up to at least Node 6.9 (LTS)?
This would allow us to use the newer ES6 syntax, such as the class construct, const, let, arrow functions, etc. This would also be timely with the upcoming major release.

Furthermore, could we update the linter to allow 2-spaces indents and 100 max line length?

[thisandagain]

I think with the work coming in as part of #128 I'd be happy to do a major version change and update the minimum engine to Node 8. Yay ES6 syntax! 🎉

I'd prefer to keep the style stuff where it is. It's a lot of work to maintain so many repos (personal, Scratch, Mozilla, etc.) and it's really helpful to have one consistent set of style rules across my stuff.

[thisandagain]

Update: with 5.0.0 we have updated the minimum engine to Node 8.0.0. Happy to accept PRs that migrate over to usage of const, let, arrow functions, and ES6 class syntax. Note that our eslint config will need to be updated at the same time.