Support FrontYAML in HTML comments

[mnapoli]

Thanks to @Giuseppe-Mazzapica FrontYAML can now use custom separators (https://github.com/mnapoli/FrontYAML/releases/tag/1.4.0), including HTML comments:

<!--
layout: "home"
currentMenu: "welcome"
-->

This needs to be officially supported in Couscous by using the new option, documenting the change, updating the changelog, adding tests probably and updating the version requirement in composer.json.

Couscous should support both --- and HTML comments out of the box.