Support wrapping output of linkAfterHeader

[valeriangalliat]

Fixes #110, also related to #100.

Usage:

const anchor = require('markdown-it-anchor')
const md = require('markdown-it')()

md.use(anchor, {
  permalink: anchor.permalink.linkAfterHeader({
    style: 'visually-hidden',
    assistiveText: title => `Permalink to “${title}”`,
    visuallyHiddenClass: 'visually-hidden'
    visuallyHiddenClass: 'visually-hidden',
    wrapper: ['<div class="wrapper">', '</div>']
  })
})

<div class="wrapper">
  <h2 id="title">Title</h2>
  <a class="header-anchor" href="#title">
    <span class="visually-hidden">Permalink to “Title”</span>
    <span aria-hidden="true">#</span>
  </a>
</div>