use lodash._reinterpolate to have ES6 delimiters, add more template tests

`lodash.template` compares the interpolate instance to `lodash._reinterpolate` to know if it should allow ES6 delimiters.  By using it as the force value, we can still have ES6 delimiters and force default settings in `gutil.template`.

@jdalton said `escape` and `evaluate` defaults will be exposed as modules on next release.