Offer link preload opt-in

[twifkak]

Opposite to #139, we could create a new link relation like

<link rel=sxg-preload as=... href=...>

which is converted to a Link header for SXG subresource prefetching, but not used for normal HTML. It would be registered here per the HTML spec.

Preferably, it may be possible to do this already like:

<template data-sxg-only><link rel=preload as=... href...></template>

We should verify this adds an SXG preload header but does not preload otherwise.

The PromoteLinkTagsToHeaders processor doesn't skip template contents. It probably should skip template contents except when the template is data-sxg-only.