Support for ES6

[marcofugaro]

mrodoob/three was recently converted to ES6, would be useful for this eslint config to include ES6 rules as well.

Example of a couple additional rules added:

https://github.com/mrdoob/three.js/blob/6a85683a5bf06e9617efd10ddc826a3e9f814b79/package.json#L41-L53