commonjs esModule flag

This adds the __esModule flag to the CommonJS transform.

This will allow interop of transpiled ES6 to CommonJS modules with other ES6 modules.

The remaining use case is allowing traspiled ES6 CommonJS to load normal CommonJS, and that is https://github.com/google/traceur-compiler/issues/818.

This will at least ensure that code being generated can work in an ES6 environment.

Review URL: https://github.com/google/traceur-compiler/pull/852

Closes #852.