Feature: support for GFM anchors

[mindplay-dk]

Support for GFM-style auto-generated anchor-tags is missing - for example, ## Opinionated in GFM generates a <a> tag with an auto-generated name attribute, e.g.:

<h2>
    <a id="user-content-opinionated" class="anchor" href="#opinionated" aria-hidden="true">
        <span class="octicon octicon-link"></span>
    </a>
    Opinionated
</h2>

Is support for GFM deliberately partial in Ciconia?

If so, it might be a good idea to clarify this in the documentation - I got the impression that GFM was fully-supported, but it appears to be partial? It would be good to list in the README not just which features are supported, but which ones are unsupported.