Incorrect section rendering when doc contains frontmatter

[xcoulon]

When a document begins with a front-matter portion, the title is rendered even in safe mode (using -s flag from CLI) and the section numbering is malformed (all numbers are prefixed with .).

For exemple:

---
draft: false
---

= User Manual

:sectnums!:

== Disclaimer

== Acknowledgments

:sectnums:

== Getting Started

=== Introduction

renders as

<div class="sect0">
<h1 id="_user_manual">User Manual</h1>
<div class="sect1">
<h2 id="_disclaimer">Disclaimer</h2>
<div class="sectionbody">
</div>
</div>
<div class="sect1">
<h2 id="_acknowledgments">Acknowledgments</h2>
<div class="sectionbody">
</div>
</div>
<div class="sect1">
<h2 id="_getting_started">.1. Getting Started</h2>
<div class="sectionbody">
<div class="sect2">
<h3 id="_introduction">.1.1. Introduction</h3>
</div>
</div>
</div>
</div>

instead of

<div class="sect1">
<h2 id="_disclaimer">Disclaimer</h2>
<div class="sectionbody">
</div>
</div>
<div class="sect1">
<h2 id="_acknowledgments">Acknowledgments</h2>
<div class="sectionbody">
</div>
</div>
<div class="sect1">
<h2 id="_getting_started">1. Getting Started</h2>
<div class="sectionbody">
<div class="sect2">
<h3 id="_introduction">1.1. Introduction</h3>
</div>
</div>
</div>