Distinguish preconnect links as SPF-specific.

Instead parsing of `<link rel="preconnect">` in order to preresolve DNS for the
target and preconnect the socket, parse `<link rel="spf-preconnect">`.  This
makes it clear that the feature is SPF-specific.

Progress on #124.
Merge pull request #155 from nicksay/rename-link-connect

Distinguish preconnect links as SPF-specific.