allow use top meta props in headerTemplate and footerTemplate

[monstress2]

Allow use top meta properties in headerTemplate and footerTemplate when export to pdf.
Use lable "{{ author }}" at headerTemplate or footerTemplate to replace them by top property "author".

Example.
Set at md file meta data with title, date and author. Then set headerTemplate with lables {{author}} and {{date}}. And export md to pdf.

---
title: "some title"
date: "2024-01-01"
author: "Me"
---
# test

Set headerTemplate setting to :
<div style='font-size: 9px; margin-left: 1cm;'> <span class='title'></span></div> <div style='font-size: 9px; margin-left: auto; margin-right: 1cm; '> {{author}}, {{date}} </div>

Export to pdf. At header:
some title Me, 2024-01-01

it my first public PR