ES2016+ Support

[jgardella]

Implement detection for ES2016+ features:

2016 features

• [x] exponentiation (**) operator
• [x] Array.prototype.includes

2016 misc

• [ ] nested rest destructuring, declarations
• [ ] nested rest destructuring, parameters

2017 features

• [x] Object.values
• [x] Object.entries
• [x] Object.getOwnPropertyDescriptors
• [x] String padding
• [ ] async functions

2017 misc

• [x] class extends null