Feat. request: inject properties into YAML front-matter

[MikeRalphson]

Is your feature request related to a problem? Please describe.

@manchuck over on APIs you won't hate slack noted that he has a requirement to inject additional properties into the YAML front-matter of the markdown output, e.g.:

...
layout: page
...

Describe the solution you'd like

A new option to merge in a YAML/JSON object, allowing overrides.

Describe alternatives you've considered

Unpicking and stitching the front-matter back together in wrapper code.