Make preconnect requests faster by avoiding DOM changes.
Make preconnect requests faster by avoiding DOM changes.

Preconnect requests are done by sending image requests to the target URL.
Before, this was done using `img` tags in an `iframe`, just like prefetching
JS/CSS uses `script` or `link` in an `iframe`.  However, this is unnecessary
overhead that can slow down sending the request.  Instead, use a simple `Image`
object to trigger the request.

Progress on #124.

scrapp-uk/spfjs,youtube/spfjs,scrapp-uk/spfjs,tchiotludo/spfjs,tchiotludo/spfjs,scrapp-uk/spfjs,theanh/spfjs,theanh/spfjs,tchiotludo/spfjs,youtube/spfjs,tchiotludo/spfjs,youtube/spfjs,nicksay/spfjs,theanh/spfjs,nicksay/spfjs,scrapp-uk/spfjs,youtube/spfjs,scrapp-uk/spfjs,youtube/spfjs,nicksay/spfjs,nicksay/spfjs,theanh/spfjs,theanh/spfjs,tchiotludo/spfjs,nicksay/spfjs