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.
Handle edge cases for script/style loading.

The edge cases handled are:
- Ensure switching names for an existing URL works.
- Ensure callbacks are transfered when switching names for existing URLs.
- Ensure switching URLs cancels previous callbacks.
- Ensure calling unload cancels previous callbacks.

Add tests for all of the above.

Closes #60.