Add Config::base_url to resolve relative links (#287)

[jaredlynskey]

Implements the request in #287.

What it does

Adds Config::base_url(...), which resolves relative hrefs while rendering:

let markdown = html2text::config::plain()
    .base_url("https://example.com/a/page.html")
    .string_from_read(html, 80)?;

<a href="/login"> renders as https://example.com/login instead of /login.

Behaviour

Resolution follows RFC 3986 §5.3:

reference	result
https://other.example/x, mailto:…, data:…, tel:…	unchanged
//cdn.example/x.js	inherits the base scheme
/login	origin + path
c.html, ../c.html	merged with the base directory, dot segments removed
#frag, ?q=1	attached to the base path

With no base URL set the output is byte-identical to before — the existing 128 tests pass unchanged, and the option defaults to None.

On the implementation

Resolution is done in-crate (src/url_resolve.rs, ~110 lines) rather than by adding the url crate. Only reference resolution is needed and this crate's dependency list is deliberately small, so pulling in a full URL parser for it seemed the wrong trade — but I'm happy to switch to url behind an optional feature if you'd prefer that; it would be a small change.

I put the resolution at the point the href is read in process_dom_node, so it happens once during tree building rather than in the decorators. That keeps every decorator and output format getting resolved links for free. If you'd rather it sat elsewhere I'm glad to move it.

Tests

• 8 unit tests in url_resolve covering each reference form, non-HTTP schemes, an unusable base, and dot segments at the root
• 3 integration tests through the normal test_html_conf path

One worth calling out: /../../etc initially resolved to https://example.cometc, because .. popped the leading empty segment and took the root separator with it. RFC 3986 §5.2.4 discards those instead, which is what it now does — there's a test pinning it.

cargo test passes (139 lib + 3 doc), cargo fmt --check is clean, and it builds with --all-features and --no-default-features.