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.
That's done in the `spf.net.resource.prefetch` function at https://github.com/nicksay/spfjs/blob/dns-prefetching/src/client/net/resource.js#L332