add ES6 support

[michaelficarra]

Only for nodes types that also exist in JavaScript.

[michaelficarra]

This implies also creating ES5/ES6 versions of isValid and errors that pass the ECMAScript version through errorsP. Then errorsP can decide which isIdentifierNameES* or isReservedWordES* to use and also do things like duplicate object key and parameter tests for ES6 outside of strict mode.

[drom]

For the errorsP(STATE) object. Would {ES6: true} be sufficient for now? Should it be false by default?

[michaelficarra]

Yeah, that sounds good.

[drom]

Shall we add second optional argument fn(node, options) to the isValid and isValidExpression functions? For the {ES6: true} being valid option.

[drom]

Related pull request: #51

[drom]

Looks like esvalid heavily rely on esutils for AST node type checking.
Where estools/esutils#20 probably need to be fixed before any further work can be done.