Support for CORS requests

[tesujimath]

In order to fetch arbitrary pages which don't have the Access-Control-Allow-Origin header set, I need to be able to pass mode: "cors" in with the fetch options, as documented here: https://developer.mozilla.org/en-US/docs/Web/API/WindowOrWorkerGlobalScope/fetch

An example is shown here: https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API/Using_Fetch

This should be a fairly straightforward addition to Milkis. What do you think?