Allow injecting frontmatter data in your .md files

[joostdecock]

The documentation of the configuration options states:

data, Object that specifies that additional data that you would like injected into your content’s component.

That sounds like a great idea, but it doesn't actually seem to be implemented.

This pull request fixes that by:

• Checking that options.data is
  • Not false (the default)
    AND
  • an object
• If that's the case, options.data is merged into the frontmatter of your markdown file / made available in your component/page.

[andrevandal]

Awesome! Could you add any example in documentation?

[joostdecock]

I'll try to find time for that

[joostdecock]

I'm merging this into a (newly created) dev branch as a sort of go-between solution as I'd like to consolidate these PRs, but feel it's perhaps best to not merge them into master without review by @apmontgomery