feat: add anchorLinkPrefix option

[ruanyf]

Allows you to add a prefix to the generated header ids, e.g. section-.

markdownIt().use(markdownItTocAndAnchor, {
  anchorLinkPrefix: 'section-'
}).render('# hello world')

//  <h1 id="section-hello-world">
//    <a class="markdownIt-Anchor" href="#section-hello-world">#</a>
//     hello world
//  </h1>'