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.
Merge pull request #125 from nicksay/dns-prefetching

Preconnect specified links (experimental)