feat(lib): support markdown excerpt

[kylewelsby]

Support Markdown excerpt.

Types of changes

• Bug fix (a non-breaking change which fixes an issue)
• New feature (a non-breaking change which adds functionality)

Description

Feature Documentation

Content excerpt or summary can be extracted from the content using <!--more--> as a divider.

---
title: Introduction
---

Learn how to use @nuxt/content.
<!--more-->
Full amount of content beyond the more divider.

Description property will contain the excerpt content unless defined within the Front Matter props.

Why?

To coincide with excerpt feature equivalent to Jekyll and Hugo

Redo of #250 #68

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes (if not applicable, please state why)

[benjamincanac]

Hey @kylewelsby,

Looks good to me, let me run some tests and it will be good to merge 🙂

[kylewelsby]

Amazing. Thanks for accepting 🙂