Subtitle layout support for blog posts

[hborla]

Is your feature request related to a problem? Please describe

It'd be nice to have support for a subtitle: field for blog posts.

Describe the solution you'd like

The ability to write this

---
layout: post
published: true
date: 2024-09-17 10:00:00
title: Announcing Swift 6
subtitle: Extended Memory Safety, Embedded and Cross-Platform Support, and Swift Testing
author: [hborla]
---

In a post heading and have that subtitle rendered appropriately under the title.

Describe alternatives you've considered

None.

Additional context

I'd love to add this for the Swift 6 announcement post!