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.
Should there be a `if (url)` check here to avoid creating img tags for empty urls?