Add Head.ResourceHints

[caseyWebb]

Hi, this PR adds a mechanism for adding resource hint link tags to the head. It covers the use case in #269 while being more restrictive than the proposed solution in #268.

Question: should a file type always be required for tags that support it? It's not required per the spec, but it's a best practice and mean one less Maybe in the API.

Let me know what you think, thanks!