Add CORS (cross-origin resource sharing) headers to Snap applications. This
enables web applications running on other domains to make requests against
another application.
Currently this package provides support for setting the
Access-Control-Allow-Origin and Access-Control-Allow-Credentials headers,
it does not yet do pre-flighting. If you need this, please open an issue on
Github and I'll fix it ASAP (otherwise I will add this feature as time permits).