Front matter support

[tarilabs]

Hi 👋

We migrated our https://www.jugmilano.it static website from Jekyll to JBake.

As part of that, we wanted to keep original post format by Jekyll with front matter, instead of just Properties as supported out of the box with JBake using the metadata header.

To do so, we specialized org.jbake.parser.MarkdownEngine in this way:

https://github.com/jugmilano/jugmilano.github.io/pull/15/files#diff-965db04910342888bfb916ee08eec67941a8b3d08e201659ed8fb0a51b97c401R32-R63

👉 If there is interest, I'd be happy to consider contributing that capability directly here upstream in JBake.
In that case, kindly provide guidance how would it be best for this to be an opt-in option maybe?

You can find more details in the migration PR if needed/interesting: jugmilano/jugmilano.github.io#15