Allow various aspects of the extension to be configurable

[markhalliwell]

Many of the CommonMark extensions have the ability to customize certain aspects of their extension, namely around classes and the like:

https://github.com/thephpleague/commonmark/blob/1.4/src/Extension/HeadingPermalink/HeadingPermalinkRenderer.php#L44

It would be nice if this could do the same.

[fenetikm]

As a starting point, what would be great is the ability to specify the prefix for the anchors i.e. fnref: and fn:. Ideally this could be left blank / empty.

[markhalliwell]

Closing this in favor of thephpleague/commonmark#484