Provide a relatively pure Ecmascript action interpreter

Without the demo [extensions](https://github.com/Comcast/sheens/tree/master/interpreters/goja#environment) in the current Goja-based interpreter. Reference this interpreter as `ecmascript5.1`? Rationale: Among other reasons, so [LittleSheens](https://github.com/Comcast/littlesheens) can provide the same thing referenced the same way.