Preconnect specified links (experimental)

When processing a response, parse `<link rel="preconnect">` tags, and initiate
connections via an `<img>` tag to both preresolve DNS for the target and
preconnect the socket.

For now, guard the feature behind a `experimental-preconnect` config.

Progress on #124.
No, and for that matter, I could just use `new Image()`, but I want to
track whether the request has already been made or not, and we already do
it this way for other types.