make reference maps extensible

[rlidwka]

This commit makes it possible to add custom handling for unknown reference labels:

use markdown_it::parser::block::builtin::BlockParserRule;
use markdown_it::parser::core::{CoreRule, Root};
use markdown_it::plugins::cmark::block::reference::{ReferenceMap, DefaultReferenceMap, CustomReferenceMap};
use markdown_it::{MarkdownIt, Node};

let md = &mut MarkdownIt::new();
markdown_it::plugins::cmark::add(md);

#[derive(Debug, Default)]
struct RefMapOverride(DefaultReferenceMap);
impl CustomReferenceMap for RefMapOverride {
    fn get(&self, label: &str) -> Option<(&str, Option<&str>)> {
        // override a specific link
        if label == "rust" {
            return Some((
                "https://www.rust-lang.org/",
                Some("The Rust Language"),
            ));
        }

        self.0.get(label)
    }

    fn insert(&mut self, label: String, destination: String, title: Option<String>) -> bool {
        self.0.insert(label, destination, title)
    }
}

struct AddCustomReferences;
impl CoreRule for AddCustomReferences {
    fn run(root: &mut Node, _: &MarkdownIt) {
        let data = root.cast_mut::<Root>().unwrap();
        data.ext.insert(ReferenceMap::new(RefMapOverride::default()));
    }
}

md.add_rule::<AddCustomReferences>()
    .before::<BlockParserRule>();

let html = md.parse("[rust]").render();
assert_eq!(
    html.trim(),
    r#"<p><a href="https://www.rust-lang.org/" title="The Rust Language">rust</a></p>"#
);

[codecov-commenter]

Codecov Report

Patch coverage: 93.75% and project coverage change: -0.06 ⚠️

Comparison is base (e781c6e) 94.60% compared to head (d0e4314) 94.55%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master      #17      +/-   ##
==========================================
- Coverage   94.60%   94.55%   -0.06%     
==========================================
  Files          56       56              
  Lines        2411     2425      +14     
==========================================
+ Hits         2281     2293      +12     
- Misses        130      132       +2     

Impacted Files	Coverage Δ
src/plugins/cmark/block/reference.rs	94.89% <91.30%> (-1.40%)	⬇️
src/generics/inline/full_link.rs	98.42% <100.00%> (-0.03%)	⬇️

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