Update regexpu’s API following the breaking regexpu-core changes

[mathiasbynens]

Re-posted from https://github.com/mathiasbynens/regexpu-core/issues/55:

https://github.com/mathiasbynens/regexpu-core/issues/49 changed regexpu-core’s API. We should make sure that these changes are reflected in the upstream regexpu project as well, e.g. in regexpu.transpileCode(code, options). This is a blocker for updating the demo at https://mothereff.in/regexpu.

https://github.com/mathiasbynens/regexpu#regexputranspilecodecode-options

cc @nicolo-ribaudo

[mathiasbynens]

@nicolo-ribaudo, since you implemented the regexpu-core changes, do you want to take this one?