Add customization examples

[akaleeroy]

Regarding #28 what would be a good example to add to the README? I've used the following to customize footnotes for EPUB ebooks:

const backrefLabel = 'back to text';

const epubRules = {
  footnote_ref: ['<a', '<a epub:type="noteref"'],
  footnote_open: ['<li', '<li epub:type="footnote"'],
  footnote_anchor: ['<a', `<a aria-label="${backrefLabel}"`],
}

Object.keys(epubRules).map(rule => {
  let defaultRender = md.renderer.rules[rule];
  md.renderer.rules[rule] = (tokens, idx, options, env, self) => {
    return defaultRender(tokens, idx, options, env, self).replace(...epubRules[rule]);
  }
})

Since renderers return a string this just does string replacement to add required attributes to the markup.

Reference: Markdown-It Architecture > Renderer

Seems to have worked, but I'm not sure if this is a good example to add. Which is why I opened this issue.

[rlidwka]

Yes, it is as good of an example as you can get with the current architecture.

Personally, I'd like to have markdown AST, transform it to HTML DOM and serialize that. Then you'd be able to write a plugin to add epub:type attribute between these two steps. We don't have that, so extending renderer is a reasonable approach.

[rlidwka]

added your example to README, thank you