Extend FragmentPlugin to handle page metadata

[lilyvc]

Currently if you use the FragmentPlugin and the fragment page has references to metadata, such as {meta.data.title} it will show the host page's title. This should be changed so that it references the metadata from the fragment page. Currently we would just advise that fragments do not contain such references.

Example:

If you edit have a host page referencing a content-fragment.mdx page that looks like:

---
title: Content Fragment
---
#### {meta.title}

You should see the title of that section on the host page be Content Fragment but what you get is the host page's title.

One for a follow up

Originally posted by @DavieReid in #354 (comment)