Enhancement: Allow custom scripts to easily update front matter

[estruyf]

If you want to run a custom script to process data and come back with an update. It would be great to have a way to quickly update the front matter of the page.

[estruyf]

String format to return from your script should be:

{"frontmatter": { "<field name>": "field value" }}

[estruyf]

It will be release in v7.3.0