fix: resolve relative links and images against the article's URL

[thebanri]

Following up on #201: this is one of the fixes from it, on Bubble Tea.

Feeds often use relative links and image sources (/doc/, next/, ../). nom converted them with an empty domain, so they stayed relative. They were neither clickable nor useful to copy. This resolves them against the article's own link with url.ResolveReference, the way a browser would.

The converter's built-in domain option wouldn't do it. It only sets the host, forces http, and gets path-relative URLs (next/, ../) wrong. Absolute URLs, mailto: and data: are left as they are. If the article has no absolute link, nothing changes.

Before → after, for an article at http://127.0.0.1:7781/blog/g1/:

See the docs /doc/ and next /next/.
See the docs http://127.0.0.1:7781/doc/ and next http://127.0.0.1:7781/blog/g1/next/.

goquery moves from indirect to direct in go.mod, because the option's function type names it. It was already in the module graph.

Tested with a table test (root-relative, path-relative, ../, fragment, //host image, relative image, absolute, mailto), which fails on master. Also checked by hand: built nom, ran it in a pty against a local feed, and read the article screen.

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