head: add link-rel tags automatically where possible + extendable

[davidsneighbour]

Add link-rel tags for all things that can be automated and add a toml-table for all other linkrelations:

• license link rel="license" href="copyright.html"
• links the archive <link rel="archives" href="https://example.com/archives/">
• links to top level resource in an hierarchical structure <link rel="index" href="https://example.com/article/"> - automatic
• EditURI <link rel="EditURI" href="https://example.com/xmlrpc.php?rsd" type="application/rsd+xml" title="RSD">
• pingback <link rel="pingback" href="https://example.com/xmlrpc.php">
• webmention <link rel="webmention" href="https://example.com/webmention">
• micropub <link rel="micropub" href="https://example.com/micropub">

see

• https://developer.mozilla.org/en-US/docs/Web/HTML/Attributes/rel
• https://developer.mozilla.org/en-US/docs/Web/HTML/Element/link

[davidsneighbour]

can be done via hooks