feat: Add support for common ES6 features
feat: Add support for common ES6 features

Node 4.0 supports [several ES6
features](https://nodejs.org/en/docs/es6/).
This updates linting to support those new features.

Two features are purposely disabled.

`objectLiteralDuplicateProperties` is a potential source of unexpected
bugs.

`objectLiteralShorthandMethods` appear hard to read.

meetearnest/eslint-config-earnest,meetearnest/eslint-config-earnest,meetearnest/eslint-config-earnest