Add CORS package to the core

[franciscop]

Is your feature request related to a problem? Please describe.
One of the basic features of APIs is to be able to work nicely with CORS. This is not possible with Server.js, and instead you need to either install a third party package or do it manually.

Describe the solution you'd like
Have the cors package be a part of the core. It should be disabled by default, but easily be able to do:

server({ cors: false });   // Default (to keep it retro-compatible)
server({ cors: "*" });
server({ cors: "https://serverjs.io" });
server({ cors: ["https://serverjs.io", "http://localhost:3000"] });
server({
  cors: { origin: "https://serverjs.io",  methods: ["GET", "PUT", "POST"] }
});

Describe alternatives you've considered
Using the cors package; it'd be nice to have it in the core so we don't need to install it each time.

Additional context
Since server.js is supposed to work nicely both as an API server and as a renderer (traditional) server, this makes making an API easier.